What Causes A Brown Discharge During Menstruation?

I just had sex 3 days ago on tuesday and that was my second time ( my first was probably a month ago) but a couple days before I had just got off my period. Now (being 3 days later after the sex which was kind of rough and dry) Im having this brown discharge that has a smell too it. I m having no pain itching or burning just concerned as what this could be. The sex was protected so it can t be pregnancy. Any explanations?