What Causes Heavy Menstrual Bleeding?

Hello I am 29 years old, 135 pounds, 5'3, have previously been on birth control patch and pills years ago. This year I have literally been on a heavy period all year. It stopped for a week or two back in April. I've been to different gynocologists over the years and they haven't found cysts or anything like that. Birth control pills haven't worked. What is your suggestion? thanks so much.