Brown Discharge

So, I got the depo shot recently and my boyfriend and I waited the exact seven days reccomended before having unprotected sex. We've had unprotected sex multiple times since then. I got the shot on the 21st of December and was supposed to get my period on the 2nd. I never got my period and starting this week I've had a lot of brown discharge and some spotting. Is this normal? Should I be concerned about pregnancy...?