i had sex, might be pregnant

so i had sex with a guy a day before my period was supposed to start and we only did it for about ten seconds before we were interrupted so he didn’t finish and he was wearing a condom, i’m 3 days late tho which is fine cause my cycle isn’t super set in stone, ik that there’s a 99% i’m not but i am just curious about fertility levels right before a period and how long you have to have sex to even get precum?