I keep getting diarrhoea and lower abdominal pain and pains down the back of my legs around the same time every month. abdominal pains feel like menstrual pains.

Likely endometriosis: Endometriosis is defined as endometrial glands and stroma that occur outside the uterine cavity. The lesions are typically located in the pelvis but can occur at multiple sites including the bowel, diaphragm, and pleural cavity. While endometriosis is a common and nonmalignant process, ectopic endometrial tissue and resultant inflammation can cause dysmenorrhea, dyspareunia, chronic pain,