I was wondering if there’s a possibility of be becoming pregnant as me and my partner has unprotected sex and he slightly cummed in me before telling him to pull out but it was the same day I started taking the contraceptive pill

yes there is absolutely a possibility. that is how getting pregnant works, finishing in someone, whether it is just a little or the whole thing. if you werent using any contraceptive & one day of the pill wont protect you

You weren’t protected, you weren’t taking birth control long enough. You should take a Plan B asap and take a pregnancy test in 3 weeks.

Be aware that even if he didn't cum inside you, there's still precum that contains live sperm. Like others said above, the pill did not protect you yet since it takes multiple days for it to suppress ovulation (unless you started it within first 5 days of your natural period). Take plan B and take a pregnancy test after 3 weeks to check if it worked.