What is this?

Ok, so I had unprotected sex a couple days before my period. Like Sunday Monday and Tuesday. Af showed up 2 days early. The duration of af was mostly normal but she wasn't near as bad as usual. I usually have bad cramps and heave bleeding. This time no cramps at all and used normal tampons. But now it has been 2 days after af has gone away and I'm having brown discharge when I wipe and very low cramps. THIS HAS NEVER HAPPENED TO ME BEFORE IN MY LIFE! Never ever happened to me. Why now and what could it be??