Doctors said I was too young to have endometriosis at 13 - I had to take morphine to cope

But the most frustrating thing for Grace, and why she contacted BBC Your Voice, is because she does not feel medical experts recognise that teenagers and adolescents can suffer from endometriosis.

Grace's GP did not diagnose her condition and nor did the first consultant she saw privately.

She says: "Even a specialist didn't believe me. And if a specialist won't listen to me, who will?" It was not until the family pushed for a second private opinion that she got a positive diagnosis.

Endometriosis, which occurs when tissue similar to the lining of the womb grows outside it, can cause debilitating symptoms, such as pelvic pain, heavy periods and fatigue. About 10% of women are affected by it, according to NHS data.

This week BBC presenter Emma Barnett revealed she had to have a hysterectomy for her endometriosis - which she said she never wanted to have.

It can develop at any age once periods begin - and it is a common misconception that only older women suffer from the condition, says the charity Endometriosis UK,

Faye Farthing, a spokesperson for the charity, says the government, NHS and health practitioners need to improve menstrual health education for young people "if we are to ensure the next generation are not robbed of the future they deserve".

The charity says it can take a long time to diagnose endometriosis - an average of nine years in older women.

And in the early stages, for example with teenagers, it often doesn't show up on an ultrasound scan. However, new tests are being developed which will help spot the condition earlier.

Getting pregnant can be more difficult - even aged 14, Grace says she has been warned it is likely to affect her fertility. She's been advised she may not be able to have children after the age of 30 because of damage to her reproductive organs.

She feels rushed: "I have to have kids within the next 15 years, otherwise I might not be able to."

Paediatric gynaecologist Dr Gail Busby says nearly 80% of adolescents will have painful periods, but clinicians should recognise when a patient's symptoms are not normal - like missing school, missing PE every month or not going out with friends.

She says: "When you're in bed and your best friend is a hot water bottle - that's not normal."

Grace's case, showing symptoms at the age of 13, is not unusual, says Dr Busby, who sees girls both younger and older than Grace at her clinics. As well as her NHS work, she has opened a private adolescent endometriosis clinic in Manchester.

The impact of symptoms is not just physical. Girls like Grace are more likely to suffer with anxiety or depression because their pain affects what is an important time for educational, emotional and social development.

"We should enjoy adolescence," Dr Busby says, "but they can't do what they enjoy doing, what their peers are enjoying doing."

There is a history of endometriosis in Grace's family, but her mum Samantha says that did not make it any easier to get a diagnosis.

"When your child's on the floor in pain, it's very clear it's not just a bad period. But professionals say 'Oh we'll just put you on the pill for six months and see see how we go.'"

Grace has recently had a laparoscopy - keyhole surgery - to remove her endometriosis.

She hopes the procedure will help relieve her symptoms, at least temporarily. She has been advised to take a hormonal contraceptive to help slow monthly tissue growth and lessen the pain.

By speaking out, Grace hopes more teenagers will have the confidence to pursue a diagnosis. "I felt I was going crazy. That it was all in my head," she says. "But actually, you know your body best and your pain is real."