Pain after sex?

After sex my vagina really burns.. we didn't use lube so that may possibly contribute and/or be the root cause of the problem. BUT it's not dry and never is dry so idk. It just worries me. It is really painful and sensitive and it hurts A LOT. 
(P.S: I know for sure its not an STI because 1. we both have only ever had sex with one another, and I know this for sure because we've basically lived together since we were young and 2. we always use a condom).
Please help! Any suggestions?❤️