The few little sperm introduced would have to be superheroes to reach the bullseye. It's theoretically a possibility as is winning the lottery. Very, very, very unlikely.
ii feel like i should not say it is impossible, but it is impossible. 1- you can only get pregnant when you are ovulating, which is not right after your period. 2- there needs to be intercourse to get pregnant. 3- it takes a lot of sperm to travel for one of them to reach the egg. but, good to be concerned and responsible. if it were me, i would not worry about it. nevertheless, i recommend getting on some kind of BC, so you don't have to worry in the future.