Is it possible to be pregnant while on depo?

I recently got started on the Depo shot. I was on several different pills and all of them made me sick and the very last pills I took gave me a long,painful period. My doctor recommended I try the depo. I have been on it since the middle of may. I just got done with antibiotics for a sinus infection week before last and then had unprotected sex about 5-7 days later. When I got prescribed my antibiotics I was told I had to use back up for 2wks to be on the safe side..so my question is..could I be pregnant? Also is it "normal" to be spotting with Depo?