I had a similar rash that appeared out of no where! It covered the back of my legs, from my knees up to my lower back. It itched like crazy, but did not spread if i touched other areas of the body. It looked just like ringworm.
I saw a dermatologist for it and was told that it was hives???? Not really sold on that dx. Anyways, i was given a shot of cortisone and used aveeno bath (the oatmeal kind) and sarna anti itch lotion. It went away in a few days. I developed the same rash a few weeks later, in the same location, and was treated the same way. I never got it again.....