brown discharge

I take birth control pills, but don't use other methods of birth control (condoms, etc). Last month I missed my period, took a pregnancy test but it was negative. This month I got my period, I finished about 17 days ago. 4 days ago I started getting a brown discharge. It seemed to have stopped yesterday, but when my boyfriend and I had sex today, the brown discharge was still there, and when I wipe when I go to the bathroom there's some on the tissue paper. I'm not expecting my period for another 2 weeks or so, but my boyfriend seems to think it means I'm pregnant. What is this brown discharge??