What are the chances?

I *ahem* reunited w/ an ex on the 4th of July. I was on an anti-biotic the week before which was also the week that I was on my period. I started a new pack of birth control on the 2nd. We were so caught up in the moment of being back together that a condom was really an after thought. What are the odds that I could have gotten pregnant and how soon can I take a test? I know it might be a little early to be worried, but it would be in everyone’s best interest that I NOT be pregnant right now.