What Causes Bleeding Before Periods?

Hi, i just recently lost my virginity and of course i bled. but it wasn't very much and disappeared after a day. well the day after it stopped i woke up and expirienced bleeding and discharge like it was my period but it was gone in a couple hours. I am on the pill and used protection and my period is supposed to start tomorrow or day after than. i am although expiriencing cramps already. can you help me figure this out?