I need help.

so I just got on the birth control pill. and I finished my first pack. didn't miss a single day. and started my second pack today. I was on my 4th day of my period. my bf and I had sex and he finished in me. this is only the 3rd time he's finished in me. and we've been together for a year. what are my chances of becoming pregnant? I know there is always a chance, but I got on the pill because we don't use condoms. any advice or experiences would be greatly appericated. thanks!