What Causes Heavy And Painful Periods?

I am an 18 year old female, I am on the pill. Last month my period was a little late, but I got it. Then about a week after I got my period, I got what seemed to be another period. Then again this month I got my period right on time, but now a week after the last day, I have what seems to be my period again except it's much heavier and much more painful, almost exactly like last month. I'm really worried. What's wrong with me?