The cause could be as simple as an allergic reaction, but if an HIV test would put your mind at ease, then you should definitely go ahead and have that done. It's a good idea for all sexually active people to get an occasional STD work-up, actually.
More often than not, it's an easily remedied issue, so I wouldn't worry overly much.
ETA: I had an additional thought: Try popping a Benadryl the next time the rash appears. If it immediately subsides, you'll know you're experiencing an allergic reaction to something. (I hope, for your sake, it isn't bedbugs
I've heard that they're the devil to get rid of. You might want to examine your sheets for small spots of dried blood, or wait until your room is pitch dark, then flip back the covers and shine a light on your bed. Those are the only methods of detecting them that I'm aware of.)
