Hi Kate, antibiotics can definitely effect 'oral' contraceptives but i am unaware of antibiotics being a reason for periods to come mid cycle.....if your talking about having one of the implant forms of contraceptives, i would suggest you consult with your doctor about your irregular cycle. It might be a one off, but it's not uncommon for young women to have irregular periods and if that is what's going on, the implant contraceptives will not help regulate your cycle the way oral contraception does.
Hope that helps.....JJ