Relax! Sore breasts are common when you're on birth control, especially if you're skipping your period. Yes, they can be a sign of pregnancy, but more often they're not. You're probably not pregnant, especially if you've been taking your pills correctly and you haven't been on antibiotics or anything like that.
On birth control that made me stop getting my period, I've used monthly pregnancy tests just to make sure. You can get the cheap ones, they all work the same.

If you're still concerned in a few weeks, take another test.