What Causes Heavy Prolonged Menstrual Bleeding?

I am a 43 year old woman on Minatrin 24 fe, and have been since October. Some months I barely bleed at all, and other months I bleed off and on all month, but rarely for more than a couple days at a time. This month, after a week into a new pill back, I started light bleeding, which got heavier after the first two days, and now it has been 8 days straight of bleeding. Should I be worried about anything?