Chance of pregnancy on period?

I've had sex very few times, and today was the first time my boyfriend actually came in me. I'm on the last couple days of my period however, and to my knowledge that makes any chance of pregnancy pretty low? I feel a bit ignorant which is why I'm posting anonymously, I'm also wondering if it's bad that I put in a tampon right after? Might that be keeping any semen in me or something? I guess I'm just kinda paranoid because pregnancy isn't something I want to deal with at all as I'm only 17, and I also do not have money for a plan B pill. Thank you for your answers 😅