Why does it hurt after having sex?

I'm 24 yo. I workout & maintain a healthy lifestyle overall. I'm with my first boyfriend & we have sex once/month according to my cycle. I get raunchy a week before my periods & otherwise I like to be a nun lol. Anyway, we both have a healthy lifestyle. But, why does it hurt for few days after having sex? I read a lot and ik it's not any infection, I pee after sex ALWAYS. I drink a lot of water too. I just don't get it why it hurts. I have blead initially 5-6 times when I first started having sex and since then there's no blood but it hurts.