Don’t know what to think

So...me and my boyfriend used to use protection all the time, then we got comfortable with not using protection. I’m not on birth control or any thing else to stop me from getting pregnant. Basically he takes it out before he’s about it and it’s been pretty ok.

So we were doing it and he told me to get off and I think I got off too late and like one shot of it got in so the min after we drove straight to CVS and got plan b and I took it in the car, about 30 min after the sex. I do know that there something called pre-cum, and the guy can’t feel it but some cum can slip in if your not careful and you can get pregnant. Now I haven’t really thought about the whole situation since buying the plan b UNTIL I realized that I’m 4 days late on my period. It could just be my cycle changing and I’m just getting paranoid, cause I feel fine, I haven’t gotten sick or been feeling different at all, or am I wrong, could there be a small chance that I’m pregnant? I feel weird walking into the CVS and buying a pregnancy test when I’m 17 but I think I’m going to get it over with cause like I just need to know, any advice could really help, thanks 😊