What Causes Brown Discharge With Clots Before Periods?

I had unprotected sex about 3 weeks ago. he pulled out. and my period was due the following sunday. i was also to start birthcontrol the sunday my period began. my period started late and was also very lite. it ended on time. and now three weeks later im still on birthcontrol and im getting brown discharge and clots and some bleeding. and im getting bad cramps and back pain. but my period isnt due for another 2 weeks. could i be pregnant? if so did i misscarry? or is all of this caused because of me starting the pill for the first time? please help.