The case could be allergy
due to exposure to some allergen. The distribution of rashes seems to be due to contact exposure with something. History of subsiding with Benadryl
also suggest the same.
Try to find out any new object that your child is exposed for these days. Something like a paints / rubber eraser / pastel shade / animal fur etc - could also be the allergen. Avoiding allergen is the remedy.
I can not rule out insect bite also.
However, visit a pediatrician
to get her examined and start necessary treatment.