What's the likelihood of me bring pregnant?

So I had unprotected sex for the first time around three weeks ago, and he finished inside me, and I've been on the depo shot for over a year and a half. And I get my shot on time each time 100%. And I don't get my period AT ALL. I don't have any symptoms, and I feel like I'm totally freaking myself out. But what's the chance I could be pregnant?