Females, red, bumpy rash after sex?

Okay, so every time my boyfriend and I have sex "doggy style," if you will, I get this red rash on my butt right where he'd be bumping into me. I'm guessing it's a rash from irritation. It's definitely not like herpes, it's clustered right together and the majority of it is red, painful to the touch bumps. They started out as tiny, flat, red spots with only a few scattered bumps, though. I'm just wondering what this is, are there any pre-sex precautions I can take to get rid of it or lessen the effects? He hasn't shaved in a little while, his hair's not long, but it's enough to be more than stubble. I was thinking maybe his hair down there rubbing against me is causing it? No clue. This used to not happen.