A woman who was in agony and "cried and vomited" after sex said she "felt like a fraud" before being given a shock diagnosis.
Olivia Charlton was only 13 when she first had "absolutely horrific period pain" which impacted her schooling because of absences, Liverpool Echo reports.
She says she was "writhing around in bed" and believed it "normal" until the pain disappeared in her later teens.
The fourth year student at the University of Liverpool said her periods had become "horrifically painful".
She said: "I'd be in agony. We're talking me being on the toilet, crying and crying and crying, sweating, vomiting in the sink, just in a horrific place honestly thinking I was actually going to die at one point, the pain was so immense.
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Andrew Teebay Liverpool Echo)"My partner, bless him, slowly got into the routine of having to bring in water, having to rub my head, telling me I wasn't gonna die from the pain. This just kept happening more and more.
"It was happening maybe once every few months, then it was once a month, and then it was every couple of weeks. And then it was nearly every single time."
Olivia of Mossley Hill, Liverpool, first heard of endometriosis in medical school and recognised some symptoms.
It affects roughly one in ten women, and some trans men and non-binary people and causes pelvic pain, heavy, painful bleeding and obstructs organs as tissue similar to the lining of the womb grows around the body.
No one knows what causes it and there is no cure.
Despite being almost as prevalent as diabetes and asthma, it takes on average eight years to be diagnosed, let alone treated, according to Endometriosis UK.
Many people also say they have had their symptoms dismissed by healthcare professionals along the way.
Olivia was "dismissed a couple of times", but she wasn't angry like "most women in this situation would be", because she was "in denial" herself.
Her constant need to urinate was because her "bladder's the size of a walnut", she joked with friends. Week-long bouts of "serious constipation" were down to irritable bowel syndrome, she thought.
Olivia said: "I was just in denial for so long, even though I knew something wasn't right."
The 25-year-old finally went to a doctor with the encouragement of her partner.
An ultrasound showed nothing, so they removed her contraceptive coil thinking that could be the cause. But the pain persisted.
She managed the pain for another eight months before booking a specialist appointment and found it "incredibly difficult" to cope, despite her partner being an "amazingly supportive person".
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Andrew Teebay Liverpool Echo)She aid: "Sex is such a big part of relationships and you're worrying like, 'Is this partner still going to want to be with me?', you know, it's hugely impactful, it makes you just feel so self-conscious.
"I felt a bit of a fraud because I only had pain during intercourse, I don't really have period pain.
"Everyone thinks endometriosis is painful periods, which it is for a lot of women, but it's not for a large percentage of us. It can be a lot of different symptoms.
"The doctor did an examination, sat me down and he said, 'Look, I really strongly think you have endometriosis, I've seen this a lot'."
She added: "When someone actually validated it and said it to me, it was a bit like, 'Oh my god, this is real, this is actually a thing, I can't just keep running away from this now'.
"Part of me was obviously relieved because I could get this sorted, I'm not going to be in this pain forever. Part of me was terrified."
The only way to definitively identify the condition and assess its extent is by inserting a camera into the abdomen through a small incision - a laparoscopy.
Treatment includes painkillers to reduce symptoms, hormone treatment to temporarily stop growth by mimicking menopause or pregnancy, or by cutting out the growths during surgery.
When Olivia had the keyhole surgery in September, the results left her "shocked" and her doctor "quite taken aback".
She didn't know for certain whether she actually had endometriosis to coming out of theatre five hours later to learn it was "everywhere".
The stage four, "deep infiltrating endometriosis" was on her intestines and rectum, her "bowel was all twisted in on itself" and was fused to her bladder and uterus by extensive scar tissue. The doctor had removed it all.
Physically, recovery was just two weeks, but coping with the mental effects has been harder.
She said: "I felt really depressed, and I felt bad about that because I was thinking, there are so many women who'd die to have this surgery, and I'm feeling depressed about it.
"It did take a really big toll, and I would say it's only really now, maybe five or six months later that I'm starting to come to terms with it.
"Sometimes I still find it difficult to think about, and knowing it can come back in the future is really, really hard. It's not just like I've gone through this and it's done, it could come back whenever it wants."
The endometriosis tissue is gone, but the pain remains, now on near-daily basis instead of being linked to sex.
Olivia copes with painkillers, deep-breathing and focusing on the things that make her "incredibly lucky".
She said she feels lucky to have had surgery when many don't and the endometriosis hadn't grown in her uterus meaning she still has the option to have children in the future.
The student, who still has the option to have children in the future, found her experience gives her more understanding when she encounters patients with endometriosis during her medical training,
She said: "There's a very bad tendency with a lot of diseases, not just endometriosis, to brush it off - 'It's not serious, it's women overreacting, it can't be that painful, it's not that bad, it's not like you're dying' - and I think that's really damaging."
People are often told painful periods and pain during sex are "a normal part of being a woman, and a lot of women get dismissed and ignored".
Olivia urged them to "hang in there" she said: "Don't do what I do and just turn a blind eye to it, because you know your body and you know something's wrong, and you need to persevere and seek treatment.
"There is light at the end of the tunnel."