Sex and pain

Me and my boyfriend had sex for the first time a week ago and then again 2 days latter and my vagina started to hurt a bit then we did it again today and it Really hurt a lot for the first couple of minutes, we have only ever had sex with each other so it can't be an STI so what else could it be?? When we first had sex it hurt a little bit  which I know is normal but it still hurts!!!!! What's wrong with me?!!