I'm not sure why a rash that comes back would be HIV- it could be a lot of different things, so I wouldn't jump to the conclusion of HIV right away. If you have been sexually active without protection- you should get tested, but don't go to worse case scenario with a rash- they happen. If it comes and goes, it could even be something like hives, which could be caused by stress. Your best bet is to go see a doctor and let them take a look- they will need to gather more medical history and take a look at the rash in person to give you an accurate diagnosis.