Brown discharge? unprotected ! ?

I had with my boyfriend on the 1st december. He d inside of me. My period is due on the 13th. I had brown discharge yesterday (5th december) but it was very little and hardly noticeable. This morning I woke up with a little patch of the brownish discharge on my panties. I go ogled it and it said something about the menstrual cycle, but I only get brown discharge AFTER my period and never before. Could u please tell me what it could be?? I can't find the correct answer. Thanks