My(22m) gf(23f) feels pain after having live sex but not when we use a conndom

So me and my girlfriend have been having a pretty good sex life using condoms. Of recent we decided to try sex without a condom, it's actually pretty good during because there's nothing like pain. The problem is that after a few minutes,she develops a burning sensation inside her vagina.We have tried to always ensure she's wet enough but it seems not to help. What could be the issue?