When did I get pregnant?

I am 13 weeks pregnant. I recently broke up with my boyfriend. We had sex the week I was supposed to have my period. I never had it. 2 weeks later, my period is extremely light, and short-3 days-as opposed to the 9 day, extra heavy periods I usually have. I never took a pregnancy test because we thought I wasn't pregnant. Anyway, he moved back to Mexico, and 2 weeks later, we broke up. 2 weeks after that, I took a test because my period was missing yet again. It was positive. So when is it most likely that I got pregnant?
I'm asking because 2 days after we broke up, I went to a party with my friends and got drunk with this guy that had been trying to get with me for a year. We had sex. We used a condom, but it slipped off and we were just hoping it wasn't his because he really just doesn't want another kid. He has 2 kids with his ex.