What Causes Vaginal Bleeding For 2 Weeks During Period?

I have been on the Depro Vera shot since January 2010, supposedly it is supposed to help have shorter periods as well as prevent pregnancy, I recently had my period about 2 weeks ago and have still been bleeding a deep red color, I had intercourse with my boyfriend yesterday and although I thought my period would have been over, there was blood on the condom. What is wrong?