What Causes Heavy Bleeding And Clotting In Menstruation?

I was a week late, and started lightly bleeding yesterday night. Throughout the day its gotten heavier and heavier. I just put a tampon in 30 min. later blood is soaking my sheets and dripping down my legs. And i am releasing clots that are the size of quarter maybe a little larger. I took 2 htp and they were both negative. one when i was 2 days late and the other when i was 5 days late. So what could this be? I Have never had heavy bleeding like this