What Causes A Painful Menstruation With Heavy Bleeding?

the day i was supposed to start my period (05/05/11) i had dark but very light bleeding. it lasted 2 days then stopped for 1. started up again with a little red and a little heavier stopped again then started again. its 05/13/11 and im still spotting. im 21, 5' and have always had a painful period with a lot of bleeding.