What Causes A Rash On The Chest And Back Of A Toddler?

Hi my 3 year old son has a rash that looks like a sunburn on his chest and back. Brought him to the doctor yesterday and they said it was viral and would go away in 3 to 5 days. This morning he woke up and it looked significantly better, but by night time after a warm bath it returned...has a very slight fever 99.5. Has eaten nothing new...no new detergents...seems to have a little cold as well. Please help. Thanks. ps He was still playing, eating and going to the bathroom normally, no vomiting or diarrhea.