Rash On Chest, Back, Heat Bumps. Taking Benadryl For Itching. Are These Associated With Upper Respiratory Conditions?

my son has a rash on chest and back, it looks like heat bumps, the emergency room dr, told me it should go away on its on in couple of days, and there is nothing to take, but benyldr. for itching. It seems as though its confined to the same areal. not in the face or legs just on back stomach and chest. Can these types of rashes be asssociated with that of colds and sneezing ,etc. upper respitory condtions,which causes break outs in the skin.