pregnancy scare

okay so a couple of days ago me and my boyfriend had sex and he nutted in me, this is the first that this has ever happened to me. but the next day my cycle came on. so moral of the story is should be i worried about possibly being pregnant or should i just relax? i’ve read hella research and all of it is saying that i shouldn’t be because you aren’t as fertile that close the your period. also i don’t even know if his sperm is even fertile. lol omg this is hard 😭