My sister has endometriosis and lots of blood clots during menstruation was what caused her to see a doctor in the first place.
It all started suddenly for her. She told me that she never had blood clots before and now she gets them all the time. She also has chronic pain around her pelvic region. It turned out that she has endometriosis. The tissue that lines her uterus grows on her ovaries and is expelled every month during menstruation.
It's kind of scary but it has improved a little bit because she is on medications. She is taking birth control pills to prevent menstruation.