What are the chances?

The condom broke when my boyfriend and I were having sex last week and he came in me. I was technically on my period but it's light and is on and off on period days so I wasn't bleeding during the sex. I'm on birth control and take it every morning at the same time and haven't missed it in at least 4 or more months. What are the chances I could he pregnant? My friend told me you're least fertile on your period, plus my birth control is spot on.