What Causes A Period With Heavy Bleeding And Clotting?

I'm a 35 year old healthy woman. I have a history of ovarian cysts which I was in the hospital for 3 days from a (laprascopy) surgery a few years back.

I had my period 10 days late last month and when it came, it was extra heavy with clotting. It lasted for 6 days like that. Now this month it's on time but it's extra heavy also. What could the problem be?