Spotting on birth control?

Brooke

I recently had sex two days ago and today I'm spotting, and my period is predicted to start tomorrow. Also, I'm on the pill and have been for about three or four months. The spotting is a bit of a brownish color. Is this normal, or could it be a sign of implantation? (I'm trying to avoid pregnancy, hence why I'm on the pill.) I'm also having bad cramps.