My dd had a rash like that only on her stomach, and the peds said that it is the only kind of rash to be concerned about and to call the doctor about. So do call. It can mean a wide variety of things, from strep throat to an allergy, but it can also go along with really serious things like leukemia and menengitis, so it is worth testing and it is worth a call. Don't get too worried - I think it's almost always strep. That's what it was in my dd's case. But the ped. will want to see it and test.