What could be the cause of having an ongoing period for a year and a half nonstop and passing large blood clots?

Multiple Causes. First of all that is highly abnormal and needs immediate investigation. One possibility is cancer of the uterus or cervix and that must be checked. Other possibilities are uterine polyps or fibroids. Despite the tubal, you could have been pregnant (1-2% chance). And it could be from endometriosis or maybe going into menopause. But very very abnormal.