Go to a Doctor on this one. It really depends on what the rash looks like, a picture would be helpful, but I've had a rash that spread quickly like that and it was Eczema. It needs to be taken care of asap, I had it when I was younger, it itches horridly yet hurts when you touch it in the more extreme cases (I know this cause I've seen a skin specialist (dermatologist) and it was one of the worst cases they had seen in their 25 years). There is a steroid cream for the main body and one for the face, both of which are highly effective. The body is called Clobetastol (or something close, I don't have my creams on hand) and the face is Desonide. I had the rash for 2 weeks and it was horridly painful and eventually became infected. With a small dose of bactrim for the infection and the steroid creams I was better with in 3-4 days.
What does the rash look like? Is it red circle like rashes? if so possiblities could point to ring worm, I myself have dealt with a fungal infection maybe you hould try this cream called LAMISIL. before trying this cream id suggest contacting a walk in clinic or your Family Doctor.
I once had a similar rash to the one u described , I got some zyrtec medicine and it really helped .