Stinging during sex

So I'm 18 and I've been sexually active for 5 months (only had 1 partner) and when I have sex it's fine for the first about 5 minutes and then after that it starts to sting quite badly. Also, after I've had sex it's painful for me to have sex again for at 12 hours? I've done an STD test so I know it isn't that, any ideas what else it could be?