Morris had heard of RED-S before, she had even written her whole dissertation on the subject at university, and was gobsmacked to discover she had developed the condition, which can be associated with restrictive eating habits.
So, too, was her mother, Julia. "As a mother, I feel terribly guilty because I didn't spot it early enough," she says now. "Issy was quite lean, but she was eating masses – she was always eating. But she obviously wasn't eating the right fuel."
Morris has since identified glandular fever in 2018, which ruled her out for two seasons, as a "turning point" in the development of her RED-S.
"I automatically thought, 'OK, as long as I don't gain weight while I'm not training, then I'll get back to racing sooner,' " a mindset she now knows is not right.
"Then when I did return to training I increased my food intake but also my training, and that was the fundamental issue, the balance was just off, but I had no idea. I was eating what I thought was loads, but it just didn't match the amount I was burning."
On her return, Morris shaved 25 seconds off her 5km personal best, finished fourth at the 2019 European Cup and was, seemingly, in the form of her life.
"That's the thing that's so scary about all of this," says Morris. "There were no warning signs. My performances were improving, so why would I need to eat more? They term RED-S in the literature as an iceberg – it's completely hidden."
The two years that Morris's body had spent fighting an energy deficit had massively disrupted her hormonal health. But because she was on a contraceptive implant, which works by chemically suppressing hormones to prevent pregnancy, there was no indication she had the condition.
Her experience was compounded by the fact that RED-S can only be detected by those trained in clinical sports science, due to there being so little awareness about it among general practitioners.
In her quest for answers, in August last year Morris sought advice from Dr Nicky Keay, a renowned sports endocrinologist who works with athletes and dancers with RED-S, who suggested she try HRT. Doctors, however, refused to prescribe it to her. One was adamant that the reason for her missed periods was due to Polycystic Ovary Syndrome, a condition commonly found in about one in 10 women in the UK which can cause irregular menstrual cycles.
On another occasion, she spent an hour-long hospital appointment with a gynaecologist who had not heard of RED-S, explaining why she should not be put on the contraceptive pill in line with the Endocrine Society guidelines which warn against the pill for women with suspected RED-S. Morris's fight to be prescribed HRT was exhausting, with bills for medical appointments over £1,000.
Aware that low oestrogen levels can be masked by hormonal contraception, Morris decided to have her contraceptive implant removed after the first coronavirus lockdown last year.
"I remember asking the specialist who took it out how long it would take for me to get a period," recalls Morris.
"He just said, 'You probably won't get one until you stop doing sport.' I knew that wasn't right, but it kind of fed the denial and the fact that I thought it wasn't as big an issue as I know it is now."
When Morris read February's edition of Women's Sport Monthly, featuring the story of former British middle-distance runner Bobby Clay, who developed osteoporosis aged just 18 as a result of long-term RED-S, parts of the interview resonated deeply. It was the catalyst to Morris getting in touch.
"I was told last year that I had osteopenia, or weak bones," she says. "It was a massive shock."
Morris is yet to find out the true extent of her RED-S, although she cannot have another bone density scan for another year because of the small levels of radiation involved with the procedure.
She admits her chances of being selected for what would be her first Commonwealth Games in Birmingham are "pretty slim", given that she is currently having to limit her training from 25 hours to eight a week. But she is determined to stay positive, knowing her osteopenia can be reversed with time.
"You've got to look at the bigger picture. Off the back of this, I'm going to be a fit and healthy, sustainable athlete instead of a potentially broken, ruined one," she says.
For now, highlighting the potentially career-ending consequences of contraception masking essential hormonal processes, under-eating and overtraining is Morris's own, personal victory.
"My goal from all my experiences is to now create a greater awareness to the medical world, sports coaches, professional support staff and to athletes of this hidden condition, in the hope it happens to less athletes in the future."
