Bleeding and burning after but no sti

So I've been having sex w/ my boyfriend for about 5 months without a condom (I'm on the pill) and for some reason after we have sex almost every time it hurts to pee after super bad it burns on my vagina and I always have redness and some bleeding on the skin at the rear of my vag, it's really annoying and I dunno what it's from or how to prevent it but I could really use some help? I've been tested and have no STIs (& we usually use spit or silicone lube so I don't think it's dryness, plus I'm only 17 so that really shouldn't be an issue right?)