Can you get pregnant while missing pills and breakthrough bleeding?

I have been on 3000mg of amox. due to a sinus infection. I am on day 15 of my birth control pills today, and started my period or breakthrough bleeding. It is much lighter than a normal day one of my period. I am a little confused on if this is a problem with getting pregnant. I am uming my pills did not protect against pregnancy if I got a period today. Anyone experience something similar. I am uming my body thinks I have missed pills.