Fran Kirby had a nagging feeling that something was amiss – the constant fatigue, nausea, and persistent brain fog. However, it wasn't until she collapsed in her hallway that she realized the severity of her condition. She regained consciousness after being slapped and was informed that paramedics were en route.

"I just got really, really extreme chest pain. Like something that I've never, ever experienced before," Kirby told BBC Sport. "I didn't know what was going on. My head started to get really dizzy and I remember just saying that I've got to go and sit down.

"I got up, walked across the kitchen and then basically just completely collapsed in the hallway, like passed out completely.

"I thought I was having a heart attack."

The following day, she received a diagnosis of pericarditis – an inflammation of the fluid-filled sac surrounding the heart. This condition typically results in chest pain and fever. Kirby's collapse occurred in November 2019, leading to a 10-month sidelining. However, she made a full recovery and comeback. The five years since her return have been fruitful for one of the most gifted players to represent the Lionesses in recent times.

She added eight trophies to her collection with Chelsea, lifted the Euros trophy with the Lionesses, and then signed for Brighton. Five months ago - shortly before England retained their European title in Switzerland - she retired from international football for good.

Having considered ending her career early because of her heart condition, the return to action has delivered more than she could have ever imagined.

"There's been times where I feel I'm not going to do this any more," Kirby said of her career. "I'm not going to do this to my family any more, I'm not going to put them through it, I'm not going to put myself through it.

"That was tough. But now that I've gone through it, it makes me proud that I've been able to get through it."

The diagnosis did not mean the end for Kirby. Instead, it marked the start of a new challenge: a long, difficult journey back to full fitness and health.

"I didn't know what to do with myself. I was agitated, I was so poorly," she said. "I would be on really strong medication, which I didn't really agree with in the beginning, but I knew it was the only medication I could take. So I was just forcing it."

She leant on support from Chelsea. "I remember on Christmas Eve [2019] I called our doctor, Francisco, who was incredible during the process, and I called him crying down the phone - I was like, 'I don't know what to do. I am so ill. I am so tired all the time.' I was sleeping 16, 17 hours a day."

Football was almost the last thing on Kirby's mind. "My thought process wasn't 'I want to feel better to play football', my thought process was 'I want to be better'," she said. "Even if I can't play football again, even if I can't go for a run again, I don't want to have a dull, achy pain in my chest."

Kirby said her illness was a "real eye-opener" and gave her a new appreciation for life's simple moments as she got better.

"Walking the dogs, that was just so exciting for me," she added. "To be able to go back on a pitch and be able to play and be able to train and be involved and going through that was a really, really nice moment, but for me the biggest moment was being able to walk my dogs."

Since her illness, Kirby, who was in her early teens when she tragically lost her mother to a brain haemorrhage, has been "a lot more cautious" about her health. She takes extra care with rest and recovery as pericarditis can occur after a viral infection, such as a cold or flu.

"I obviously had got ill and then came back too early to try and play, and then it just kind of attacked that specific part of my body," Kirby said. "I hadn't been feeling well for a while, but I kind of just put it down to... I'm a footballer - I'm going to be fatigued, I train every day, I go to the gym, I'm going to be tired."

Kirby emerged as a youngster at Reading, scoring at a rate of more than a goal a game, a supremely talented forward who earned England caps and played in a World Cup while with her hometown club, before joining Chelsea in July 2015.

Her recovery from the hallway collapse was carefully managed, and Kirby returned the following season with a vengeance - scoring 16 goals and assisting 11 more in just 18 league matches for Chelsea. Those performances earned her the PFA's Players' Player of the Year award.

These days, she is particularly careful if she feels her body needs a break. "I wouldn't say I'm scared, but aware. I don't want to have that pain in my chest ever again. It was terrifying," said Kirby, who is now in her second season with Brighton. "So whenever I get ill. [When people say] 'Fran's not in again', I'm like, yeah, because I've gone through that. I don't want to experience that again. I need to allow my body to recover."

But even with a more measured approach to her football, Kirby - who was nicknamed 'Mini Messi' by former England boss Mark Sampson - still feels the pressure. "Everyone expects a certain level when I play," she said. "And that comes as part and parcel of obviously playing for Chelsea, playing for England, winning all of these things.

"You know every time someone turns on the TV or they go to a game they expect Fran Kirby to be this Fran Kirby. Fran Kirby can't have an off day."