D.W.
You really shouldn't attempt to treat anything without knowing the cause of it. While most rashes are viral (can't do anything to treat a virus), some can be bacterial, related to allergies, related to contact with a pathogen, etc.
Your best bet is to see a dermatologist and at the very least your family practitioner. However, I'd always take a dermatologist for a skin-related issue as their residency/fellowship really trains them for being able to identify small differences in the appearance of rashes that can help determine the cause.