So this happened twice now...

So i've been with my SO for almost 6 months now, i went to the doctor and got tested before we were sleeping together and he had only ever been with one other person, so i'm not worried about an std or anything. However, when we had sex last night i had to pee after and it burned a little bit, and when i went back to bed it was burning for a little bit down there but went away eventually. This has happened before, but didn't happen the last time we had sex. Is it a UTI? Yeast infection? help! what should i do?