Pregnant?

I’ve been on birth control for 3 weeks (started my fourth week this week). While vacationing with my boyfriend, we had unprotected sex and he came inside me. This was the third time he came that day so it wasn’t very much but I am very nervous that I could be pregnant. What are the chances I could be pregnant since I’m on birth control and take it every day at around the same time and haven’t missed a dose? And if the chances are high that I could be pregnant, is it too late to take a plan B (this occurred on Sunday at around 1 AM)?