Endometriosis is the most common cause of very painful menstrual periods.
After a woman starts to menstruate each month, the inner lining of her uterus starts to thicken in preparation for a pregnancy. If she does not become pregnant, she sheds the entire inner lining which passes from her body. Endometriosis occurs when the inner lining of the uterus attaches to other parts of the body, such as ovaries, intestines, bladder and even cervix or vagina. Then, each time she menstruates, she bleeds and hurts. For example, if the inner lining of the uterus tissue is attached to intestines, she bleeds into her belly and suffers terrible belly cramps.