For Ainsley Zammit, dealing with the pain of endometriosis isn't the worst part of this "hidden illness". It's that no doctors believed how sick she really was.

From the age of 13, Ms Zammit suffered cramps so severe that she would have seizures from the extreme pain.

Striking her abdomen each month, she began to realise that this was something much more than regular period pain.

"I always had period pain, but as I got older it got much worse," Ms Zammit told news.com.au.

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"I went to boarding school, so I had a lot of friends from the country. I was out for a nice day of riding horses when I got really bad period pain. It got so bad, I reached a point where I started vomiting everywhere. The pain was so excruciating."

Endometriosis is a disorder in which tissue similar to that in the lining of the uterus, grows outside of the layer. Embedding itself in areas such as the vagina, cervix, bowel, bladder or pelvis, it often then develops into painful cysts — commonly resulting in extreme abdominal pain, infertility, nausea and irritable bowel symptoms.

There is no cure for endometriosis, and even with surgery, the chance of the disease recurring within five years can be as high as 40 per cent, according to the Cleveland Clinic.

Despite these intense symptoms which caused her to miss weeks of school, it took years for the now 21-year-old to be officially diagnosed with the condition. As her parents took her to doctor after doctor, often the response would be that the condition was simply "all in her head".

"I spoke to doctors who said it was in my head," Ms Zammit explained. "I started getting my periods when I was about nine, but I didn't get diagnosed with endometriosis until I was 15.

Ainsley with her dad. Photo / News.com
Ainsley with her dad. Photo / News.com

"The whole time, doctors would say I should see a counsellor, or it was all in my head, then they decided to take my appendix out.

"It's so frustrating and you feel like an idiot, because you know something is going on but no one will listen. My family see the pain, and they didn't know what to do."

For women such as Ms Zammit, the invisible illness is a lifetime sentence that affects both her physical and mental health.

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"When I am in really bad pain, I faint and I have a fit," she said. "I was in the middle of Croatia once, and I had a seizure in the middle of the city. It's so scary.

"The pain starts before and during my periods, and you don't want to move. You can put yourself in a pretzel position to help, but it feels like your uterus is twisting — like someone is stabbing you repeatedly."

Ms Zammit, who has had four laparoscopy surgeries since she was 15 and three intra-uterine devices (IUD's) to help treat endometriosis, she still suffers from excruciating pain and bad mood swings.

"This has definitely impacted my life," she said. "It's stopped me from doing things and taking up different job opportunities.

"Now, my period will last at least seven days, and that's seven very heavy days. But my last period for example, went for two weeks, had a break of 2 days, and then came back again for five days."

Ms Zammit said she hopes a cure is found soon, as the condition not only impacts her — but those around her.

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"When I have a fit … it's very scary for people that don't know me or understand what is going," she said.

"Apparently I nearly bit my tongue off once. People thought the fits were because I'd taken drugs, so it's really embarrassing."