Paranoid

my bf and i use a condom and he pulls out every time, but occasionally he will just go in for a couple of seconds unprotected and pull it right back out, and then put the condom on. we make sure to wipe whatever precum off beforehand, but i still am nervous that something could’ve gotten inside. my period is 8 days late and i took a test when it was about 3 days late and it was negative, but i am still anxious about it. what are the chances i could get pregnant from that?