Is it normal to have a period after starting antibiotics while being on birthcontrol (complicating read detail?

My girlfriend has been on birth control for over a year i know for sure probably longer but she started antibiotics and then went on vacation for a week,she had and left her antibiotics at home and while on vacation she had her period and she came back and started her antibiotics again and while she was on or shortly after she had another period and we had sex before she left (i wore a condom, i do every time even when she is on bc) and when she came back about a week later we had intercourse and later that night she blotted a little bit and then today a week later we had intercourse and shortly after she bled a lil just like two blotches and a lil piece of skin came out on me. ( i wore a condom both times) and im just wondering if that's normal or if i need to worry? or what me and her don't know?