What Does Brown Spots In Periods Imply?

My last period was April 16th. The period I am on now started like the May 14th. But I didn't know I was even on it until I washed down there and looked at the rag and there was brown on it. Since then [todays May 18] Its been the same way until today it finally came and showed up on a pad and there was little over a quarter amount and it was super dark... I've had sex 3 or 4 times this month with my boyfriend [if that is any help] ........ what could this be????