What Causes Blotchy Rash On Legs And Stomach Of A Kid?

My toddler gets a blotchy rash that comes and goes always on his legs and stomach but his faces gets red sometimes too.. I give him Benadryl and it will go away but comes back.. I thought he was allergic to anti biotic however he has been off it a week and he still has it l.. Not sure if I should take him to the dr or wait to see if it gets better