STD??

Really my only symptom is burning in my vagina, both when I pee and when I don’t. I feel like I have to pee a lot but when I go nothing comes out. I’ve had it for almost 3 weeks now. I’m concerned because my boyfriends ex was... for lack of a better term, a hoe. She slept with a LOT of guys in a very short time period, all ones that she knew (that I know of). My bf says she didn’t have anything and neither does he, but why am I having this burning feeling?? It goes away sometimes and then comes back. I get it often after sex... is it just a UTI or could it be an STD or STI?? He’s the first person I’ve ever slept with, so I know I didn’t get it from anyone else and he says he doesn’t have anything. Help???