It sounds as though it may be hives, which come and go the way you describe. Despite their allergic reputation, the vast majority of hives are not allergic--they come for no reason and go away the same way. While you're waiting to see a doctor, you can use an over-the-counter anti-itch lotion containing camphor, menthol, or pramoxine. As for HIV, I see no reason for you to worry about that. If the rash is still there after the weekend, let a doctor check it out.
Best.
Dr. Rockoff