Pregnant after depo shot? First time user! Please help?

I received my first depo shot on June 15, I hadn't had a period before the shot and have yet to have one...I foolishly had unprotected sex and my boyfriend did ejaculate inside of me....could I possibly get pregnant from this or had the shot kicked in? I took.a test before the shot so I knew I wasn't then but the whole loophole with the time you get the shot as it relates to your period worries me...of course I'm goin to take a test but it happened June 18 so I know that's too early....but any advice or past experience knowledge would be greatly appreciated. Thanks