Okay, well there’s an easy answer and then a more complicated one.
Easy: Itchiness is common when pubic hair is growing back. You said you have none, so that means you shave or wax. Your hair starts growing back very quickly and it usually gets itchy.
Or it could just be dry skin! Like you would have anywhere else.
Complicated: You’re right, it could be an STI, a bacterial infection, or a yeast infection. So make an appointment with your doctor. Read here about another possible cause. Unless you have the results of your boyfriend’s STI screening in your hand that proves he doesn’t have anything, then I’m sorry, but you still have to consider him a risk.
Either way, if it concerns you enough to ask me, then it means you need to talk to your doctor and figure out for sure what’s going on down there. Whatever it is, you need to know for sure in case you require treatment. I’m not a medical professional so I can’t help you here.