What Causes Delay In Periods, Cramps And Dark Brown Discharge?

Hi. I havent had my period for two months now. My last period was may 18, 2011. Had unprotected sex and today I found darl brown almost black discharge in my panties and when I wipe, its a light pinky red color. Got light cramping on and off for the past few weeks and I dont know what it is. Im 23 and this has never happened. Took two depo shots back at the end of 2009. Didnt think I could anylonger get pregnant. Can u help me? I want to lnow whats happening.