What Is The Cause Of Heavy Bleeding?

The guy I have been seeing and I have been fooling around recently. A few nights ago I experienced a little spotting immediately afterward. Two nights later I experienced much heavier bleeding for about 4 hours afterward. The second time I felt him scrap my vagina wall, even though his nails were trimmed. I also recently switched birth control to Minastrin and am close to the end of the pack. Should I worry now, or see if it happens again?