Antibiotics make birth control less effective. This could be the reason, as basically it is like your body thinks you stopped taking your pills when you started taking your antibiotics and therefore your period begins. That said, I used to take ortho tri cyclen
lo, and because of the low dose of hormones it contains I would start bleeding if I just didn't take my pill at exactly the same time every day, let alone if I missed one. If it doesn't even out after taking it awhile, or you don't want to wait, you need to switch to a stronger birth control pill with a higher hormone dose.