Drug companies caution that antibiotics can decrease effectiveness of birth control for legal reasons. There have not been any studies that definitively prove antibiotics decrease birth control effectiveness.
The theory behind antibiotics increasing the chance of unwanted pregnancy while on birth control is two fold: 1) some antibiotics increase liver metabolism of the birth control hormones which decrease circulating levels and 2) some antibiotics decrease gut flora (some of which can convert estrogen byproduct into estrogen) which results in a decrease in circulating estrogen.
The above theories in reality contribute to only a very slight decrease in circulating birth control drugs. Antibiotics will not cancel the effects of your birth control; there is a very very small risk of unwanted pregnancy especially when it comes to penicillin.