Help me please !!

Hi! my boyfriend and i had unprotected sex on Monday but it was for a short period of time and he did not finish, i am supposed to get my period today but haven’t. I am not too worried because i am young so my periods are kind of irregular. i have read that it’s very uncommon to get pregnant if you have sex right before your period. is it common to get pregnant off of pre-ejaculation? i really just need reassurance and help because i am so scared.