Stinging after sex

Cyan

So me and my boyfriend decided to have sex on my period. I’m only going to share what position for the sake of getting help. I’m not sure what it’s called but I was on my stomach and he came in from behind. After we were done, I went and put a scented light tampon in. But it burns and itches when I walk like a bad yeast infection. Help?

And he hadn’t had sex for almost 3 weeks if that has anything to do with anything.