Antibiotics and Birth control?

I am currently on the pill (yaz) and have been for quite some time. I had a uti and was on macrobid for it. My boyfriend and i had unprotected sex not long after i had started taking the antibiotic. I know that antibiotics can effect birth control, but i'm just worried about it and wondering what the chances are i could be pregnant. I take my pill religiously, and i've started taking my placebo pills and have yet to start my period. They put me on another antibiotic about a week after i finished the macrobid for a yeast infection, but my boyfriend and i have not had sex since i took it, which was on wednesday. I have heard that antibiotics can make your period lighter, especially while on birth control, and mine is usually light to begin with, so i'm also wondering if that may have anything to do with it? i have been cramping but other than that i have not started yet. I'm extremely worried.