Rashes?

Ma

Hi! So, I have been noticing that after sex I would break out in rashes. It’s like little red with small white spots all over. It’ll eventually go away after 10-15 mins but, it always happens after sex. My boyfriend likes doggy style so, it’s always my butt that’s breaking out. I recently got off birth control about a month ago too. I’m not sure, if it’s just my body changing or what’s going on. We started using condoms since I been off birth control but, not always. We switch on and off. And both times I still breaking out. Can anyone tell me if this happened to them before? Any advice or suggestions? Thanks!