Hi I'm On The Depo Shot For The Very First

Hi I'm on the depo shot for the very first time. I just got my first injection about a week ago. A week before I got the shot, I had unprotected sex. Now I'm wondering if I'm pregnant or maybe it's just the shot. I was supposed to start my period within the past few days but haven't yet. Can the depo kick in this fast or am I pregnant?