What Causes Red Brown Discharge And Stomach Cramps?

hi,I am 22 5"1' and weigh about 200lb. I was supposed to start my period on friday of last week and didnt, but have never been consistent with my period so assumed it was normal. I am on BC and two days ago i started getting dark red to brown discharge only when I wipe. What could this be? its not flowing at all like a normal period but i still had cramps.. but now its more a muscle spasm in my side. What could it be?