Condom slipped off inside of me

Yikes. After my boyfriend came in me, when he was pulling out, he did not hold on yo the base of the base of the condom so it stayed inside of me and slipped off his penis. The base of the condom was outside of me (so the entire condom was not just loose inside of me) and it appeared as if all of the cum was in the condom anyways. I know this is not great and that there could be a slim chance of pregnancy, but I’m supposed to get my period tomorrow so should I just wait to see if I get my period or get a plan B? I am 17 and not on any other birth control so I really cannot afford to be pregnant.