"Nobody can tell," she told ESPN. "When you're having a bad day, they think, 'Why is she struggling today?' People are going 'come on Soph, run,' and I'm thinking, 'I can't.'
"They just won't know. They'll think, 'She's being lazy, she's unfit,' but no. I'm dealing with something that nobody else can see."
With physical injuries, "You know what it is, you have scans, and you know what to do to fix it.
"With this, it was so different. It was unknown."
In September 2021, Hillyerd was embarking on the beginning of her professional career, signing with Charlton Athletic from Manchester United in search of senior first-team football. A COVID outbreak impacted her and a large group of her teammates prior to the season starting. It was when the keeper returned to training that she realised something wasn't right.
"I was breathless, but it wasn't until three sessions in that I realised 'this isn't normal,'" Hillyer recalled.
"I'd heard of people struggling to return to play but there was nothing quite like how I felt with my heart going so fast. My heart rate got up to 224 [beats per minute], which is ridiculous."
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For context, Hillyerd's maximum heart rate at the time should have been 202 BPM. Despite her symptoms, the tests on her heart came back normal. At first glance, there was no physical reason to discontinue training. However, she soon knew that the situation was untenable.
"I did a session all about crosses. That's quite low intensity," she explained. "I'd fallen on the floor and I stood back up and I just thought 'God, I'm going to faint, I'm going to fall back over.'
"It was more than light-headedness. The dizziness -- I couldn't stand up without feeling faint. I knew that something wasn't right.
"As I was sitting in bed at night, I'd just have this excruciating chest pain and there was no explanation for it. It was so hard not knowing what's wrong and it being such a horrible thing, something to do with your heart."
The lack of answers is a common theme among patients who have Long COVID. In a Pultizer Prize-winning explainer on Long COVID by Ed Yong, patients describe the dismissal or disbelief of their symptoms by friends, family and even medical professionals. However, Hillyerd spoke glowingly about the medical team at Charlton, led by Dr. Chris Schoeb, and the way that they continued to pursue answers in the face of inconclusive tests.
"He never gave up," she said of Dr. Schoeb, and he helped her book an appointment for the specialist to confirm his suspicions of a condition known as POTS.
Postural orthostatic tachycardia syndrome -- POTS -- has some tell-tale signs. They can include dizziness, brain fog, chest pains, fatigue, and a heart rate that increases by over 30 when standing. It is also a common diagnosis for people experiencing Long COVID. Most people have never heard of the condition, let alone know what the signs are, and can go months or even years before diagnosis.
Exercise physiologist Carlo Sibulo works at the Wellness Station in Melbourne, Australia. He specialises in Chronic Fatigue Syndrome and Myalgic Encephalomyelitis, collectively known as ME/CFS. POTS is known to be related to ME/CFS.
"POTS relates to intolerances to postural changes and an inability to regulate blood volume," Sibulo told ESPN. "However, both POTS and CFS can originate from the same root cause, and hence can share risk factors."
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ME/CFS and POTS impact a variety of people. They are far from exclusive to Long COVID -- they precipitate from other viral infections as well. However, over the last 12 months, there has been a substantial increase in Long COVID cases in Sibulo's clinic. The most common symptoms that his patients are presenting with are fatigue, concentration, memory issues and exercise intolerance. He explains that this leads to "significant impact to daily life and function."
Risk factors for developing illnesses like ME/CFS and POTS include being exposed to a demanding or stressful environment, having high activity levels, and certain personality types including high achievers and carers. While anyone is at risk to developing a post-viral syndrome, these risk factors will inevitably capture a high number of athletes -- something that sports organisations are slowly becoming more aware of as effects of the COVID-19 pandemic continue to reverberate throughout the world.
Hillyerd's eventual POTS diagnosis provided her the answers that she was so desperately seeking, and an eventual path back to football.
While the science remains unclear as to the exact prevalence of POTS in Long COVID patients, there does appear to be a clear link. The severity of symptoms can vary. Some POTS patients report having to stop work or education due to their condition. However, the vast majority report an improvement of symptoms with treatment. Many do recover, either fully or to the extent that their quality of life is no longer affected.
Treatment options include increasing intake of salt, staying hydrated, and wearing compression garments. They can also include certain heart medications to help with low blood pressure. Hillyerd immediately began to notice improvements after implementing the recommendations from the specialist.
"It probably took about two to three months to get back into training since I had that appointment," she said. "It does sound long, but after having the unknown it was great for me."
There were several modifications required once she did re-enter the professional training environment.
"In training, I wore compression on my legs to help the blood come back up and the oxygen flow," she explained. "I also wasn't allowed to do isometric exercises, where you hold -- so I can't do a wall sit for example, because it just triggers it.
"If my blood is in the wrong place, it will cause me to feel faint."
It was with the help of these treatments and modifications that Hillyerd has recovered, to the extent that POTS no longer hinders her ability to perform as an elite athlete on a day-to-day basis. She is now fit and firing at West Ham, ready for her opportunity in the FA Women's Super League.
POTS, like many of the other ways that Long COVID can present itself, is invisible. This means that someone with the syndrome can appear perfectly lucid to the world around them while having any number of symptoms at the same time. It can lead to misunderstandings as to how severe an illness it is.
While Hillyerd emphasised the understanding of the coaching staff at both Charlton and West Ham, it is something she admits is enormously difficult to explain.
"Some days after I've trained really hard I can't move from the bed, I can't get up without feeling poorly. And there's other things that people don't talk about -- you get stomach aches, headaches, chest pains. The girls around me will have no idea," she said.
"You just have to kind of manage it yourself and just remember that it might just be today and tomorrow might be better."
The unfortunate reality is that more stories like Hillyerd's are likely to surface in professional sport over the next few years. Sibulo, however, offers a hopeful prognosis.
"In my experience, the prognosis for POTS is mostly positive," he explained. "The key is getting diagnosed appropriately, and to be assessed and given a tailored management plan."
Post-viral conditions like ME/CFS and POTS can be managed and overcome, as they have been for Hillyerd -- but they cannot be when clubs are unaware of what to look out for.
It is astonishing to think that Hillyerd is only 19. While she is yet to play in an official match for West Ham, she speaks as if she is already an experienced professional, and the club's trust in her speaks to her ability between the sticks. Taking into account that goalkeepers are afforded more years to mature than others, Hillyerd has a good chance of becoming a household name in English football.
"Of course I want to go to World Cups and play in Euro finals and things like that for England," she mused, "but at the moment I'm fully focused at club and doing my best for West Ham, especially after the rocky year I've had."
It's difficult to overstate the mental and physical challenge of an illness like POTS. Some days, those with the syndrome feel like they're on top of the world, that they can do the same things that they could pre-illness. Then there's the crash -- they do too much, and have more severe symptoms as a result.
Much of the battle with recovery from POTS is learning to manage those symptoms. It is learning when to stop, and having systems in place that allow you to rest and recover when it is needed. Hillyerd emphasised that management throughout our discussion, and how much that has improved for her over the past 12 months. "Bad days" today are far removed from the "bad days" of the past.
However, recovery from such a complex illness will rarely be linear -- and that's okay. Medical professionals like Sibulo emphasise the importance of keeping hope.
"You are not alone. There are many others with POTS or ME/CFS who have been able to return to meaningful athletic participation," he said. "Respect your symptoms and learn to listen to your body and limitations."
Stories like Hillyerd's serve as an example of what is possible. It is hard, and the journey may be long, but it is possible for careers and lives to return back to normal during and after the effects of Long COVID.
Most of those suffering through post-viral conditions aim to return to basic levels of activity -- walking the dog, or mowing the lawn. It provides an enormous amount of optimism knowing that there are examples of elite athletes like Hillyerd who live with the condition relatively unhindered. She gives a forthright message to others going through the same journey as her -- don't give up.
"Don't let those bad days get you down, because I certainly did. I let them get me down -- but I learned not to," she said. "You have your own techniques. It may take ages to find them, but find what works for you, because they're not going to work for everyone.
"Take it day by day. Be resilient. You will get there."