What Causes Brownish Vaginal Discharge After Having Unprotected Sex?

Okay, so I recently had unprotected sex three days ago with my boyfriend. The first day after having sex my body was sore, on the second day I had a random cramp like pain in my lower stomach and today on the third day I noticed I had like a brownish colour of discharge? and I have already had my period at the beginning of this month. I'm just wondering if that could mean?