Confused

So I started my period on September 11th. I ended it on Septemeber 18th. Starting yesterday morning September 23rd I wiped and got a brownish bloody discharge. It wasnt alot. Probably about a pantry liner amount. I still have a little today. But it lightened up compared to yesterday. I have never has this happen. My boyfriend and I had sex on the 19-21st. Anyone have any ideas on what this could be? The more it lightens up the more crampy I'm becoming.