Taking antibiotics does lower the effectiveness of the birth control pills when taking them. That's why they recommend using a condom or some other form of birth control while you are taking antibiotics. It doesn't completely reduce the effectiveness, though. Contine taking your pills for the rest of the month. Finish them out and wait for your period. That's really the only thing you can do. If your period winds up being late, take a home pregnancy test.
I just read your post again. I don't understand something. You should have gotten off for a week for your period (or taken the sugar pills that week). Then you should start on your new pack. Are you saying you had sex during that week that you got off for your period? Chances are, if you had sex during your period or even shortly after you're probably safe. There's no gaurentee though. Usually ovulation happens 14 days or so after your period. Like I said though, everyone's different and there's no gaurentee. Unfortunately, you're not going to know for sure until you get your period. Good luck.