Macular rash on child that comes and goes. What is it?

Viral or allergic. Fleeting rashes are often allergy, heat , or viral. It is very hard to know without seeing it so i would suggest taking a picture of it and making an appointment with your pediatrician/physician. Taking a picture is very helpful since, like cars with funny noises that are gone when you see the mechanic, rashes can disappear when you see the doctor.