Females, red 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 just flat, tiny red spots. There are a few of the red spots that are larger, bumped up, and sore to the touch, 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.