A boy and I tried to have sex but he didn't ejaculate in me. I'm worried about his pre cum. I got my period the day after we tried could I be pregnant?

Period endsguesswork. The presence of a period ends the possibility of guessing if you are pregnant or not.(You are not). Sex without contraception is a "risky" behavior and sure produces unnecessary "worrying". So give some thought to ... yep your guessed it, contraception in the future.
Here are some... Clinically, the odd to conceive is next to zero due to ovulation occurs some 2 weeks before period. Nonetheless, do HPT in a wk after missing period & repeat it in another wk if the first test was (-) or still in doubt; this is the focus on worry about pregnancy. However, if the boy is minor (