Pregnant on shot?

I got a deposhot, which acts like birth control, about three months back and the doctors informed me that my period would still continue since it was only my first time getting the shot, however my stomach has grown to look 18-20 weeks pregnant(which is what I would be if I didn't get the shot, from having unprotected sex prior to the shot) I have been having my period but is there any possible way I could be pregnant ? I need to know before I reknew my shot.