Butt rash??

Okay, so my boyfriend and I have sex from behind quite a bit and he’s trims regularly so it’s not like his hair down there is prickly, it’s soft but I’ve noticed I have little bumps and hard red bumps on my butt all the time now and I was wondering if anyone knows how to get rid of them? I’ve tried using some of my acne cleanser on them and that didn’t work and I’ve also tried using some witch hazel toner as well. Any advice ?