Endometriosis isn't just painful periods, it's a chronic condition in a league of its own.
One in 10 women has it yet, in the UK, it takes on average seven years to get it correctly diagnosed by a doctor - something experts want to change.
With endometriosis, tissue that behaves like womb lining is found in other bits of the body, causing nasty symptoms.
Amelia Davies was 12 when she got her first period. She soon came to dread her "agonising Auntie Red".
"At times it was so bad I couldn't go to school. I missed loads of days. The pain was really intense, with lots of different types - stabbing, cramping and burning. I was so bad I couldn't walk or get out of bed."
New guidelines for the NHS aim to reduce delays in diagnosis and save women years of unnecessary distress and suffering.
Amelia first explained her symptoms to her GP and then a few different doctors, but they couldn't find anything wrong.
"Finally, they agreed to send me to hospital for an ultrasound scan.
"So, there I am sitting in the hospital waiting room in full school uniform with dad laughing and joking about to try and keep me calm. It felt like people in that waiting room were giving me dirty looks, and assuming I was there for a pregnancy scan or something. I felt judged."
The scan revealed she had a cyst on her ovary, plus endometriosis.
When a woman with endometriosis menstruates, the misplaced womb tissue bleeds too, causing crippling pain and some rather unusual symptoms.
Some women pee blood at their time of the month. Others even cough up some blood if the rogue tissue is in their lungs.
Over time, the bleeding can irritate the body and lead to scarring or adhesions - tough cords of fibrous tissue that can cause more pain and make organs stick to each other and cause complications.
Amelia's doctor advised her to take an oral contraceptive pill to help alleviate her symptoms.
She currently takes the mini pill and hasn't had a period in two years.
But Amelia, now 18 and living in south London, says her endometriosis still causes her daily pain. She's been writing a blog about her experiences.
"Sometimes it can be really bad still. I get flare-ups and that's really difficult.
"I get the phrase, 'At least you're not dying,' quite a lot.
"I know it's said most of the time by good friends who are trying to be nice and reassure me. But endometriosis for me is the prospect of a long life full of pain. I sometimes feel like people are measuring my level of struggle against others' and that doesn't feel fair. It's daunting."
Caroline Overton, a consultant gynaecologist who helped write the new guidelines from the National Institute for Health and Care Excellence said: "There is no cure for endometriosis, so helping affected women manage their symptoms is imperative.
"As one of the most common gynaecological diseases in the UK, it is vital that endometriosis is more widely recognised."
Emma Cox, from Endometriosis UK, said: "The impact a delayed diagnosis has on a woman's life - her education, work, relationships and personal life - can be huge. On top of coping with the disease itself, women have to put up with being told, sometimes for years, that what they have is 'in their heads' or 'normal', when it isn't."
- You can't "catch" endometriosis and nothing you did made you get it
- Women and girls of childbearing age and of any ethnicity can have it
- It's a long-term condition that can be difficult to diagnose at first
- Common symptoms include pelvic pain, period pain, tiredness and pain during and after sex
- Your GP may not be able to see any signs that you have endometriosis and some tests may not show up the problem
- You may need a procedure called a laparoscopy (where a surgeon passes a thin tube with a camera into your body through a small cut in your skin) to confirm the diagnosis
- Medication (including the contraceptive pill) is available that can help control the pain and symptoms
- Some women have surgery to get rid of some of the tissue or a hysterectomy to remove the womb
- Women with endometriosis can still try for a baby - it's estimated up to 70% of women with mild or moderate endometriosis will be able to get pregnant without fertility treatment