Actually, your chances were pretty high. For one, having sex right after her period is actually more risky than doing it before. You'd think it'd be the other way around, right?
Anyway. It's great that nothing went wrong, and really your chances weren't too bad, considering you did try to keep the unprotected sex during her period. But I have a friend who went years having unprotected sex with her boyfriend and never had anything go wrong.
Life's just screwed up like that. You're lucky
When she starts the pill, don't forget to make sure she takes it at the EXACT SAME TIME EVERY DAY. It's really important, especially if you plan on having unprotected sex while she's on the pill. Same time. Every day. No fail. Don't forget. ^-^