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

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."