go see a dermatologist. There is a skin condition for your symptoms, but I can't think of the name right now. It can be treated with an antibiotic and a special lotion for the rash. Also, heat to the skin and any redness can also indicate cellulitis, which is dangerous and needs to be addressed in a hospital setting! Go get checked out!
LOOK FOR A SMALL BITE SOMEWERE...SEE THE DOC SPIDERS WILL BITE AND YOU CANT ALWAYS TELL
GET TO A DOCTOR. Could be anything, but if you have a rash moving that fast, you need to see a doctor immediately.