Pregnant? Depo Shot.

I'm on the Depo shot. Have been since May. It's now June and me and the boyfriend have sex regularly and never use condoms since I'm on the shot. It wears off July. I go back July 17th my period in May was normal. This month it's late, and I'm currently spot bleeding very very lightly. Have been for 2-3 days now. Should I be worried? Who else has had the Depo. Yes I know it works differently for everybody but I'm just curious.