Ds had a similar thing. It started on his chest but would then fade an reappear elsewhere, and then fade and reappear again in the original place.
The gp said it was an allergic reaction to a virus and that it could last a while. It lasted 3 weeks in the end, coming and going all over. It was worst on his face, and we got some very odd looks when out and about but he wasn't contagious or anything.
I took my son who is 8 to the local 'walk in centre' this morning with similiar symptoms, over the last week or so, he has had itchy blotches over his back and legs, which when scratched, goes into one big red blotch, this morning though he had some red rash type spots on his face ears, neck legs and arms. The nurse said it was an allergy and recommended piriton. Prior to the rash starting last week he did have a bit of a temperature, headache and cough, so could well be a virus type rash.