D.W.
Honestly, dermatological reactions are some of the hardest to diagnose since they all look a lot alike and often take the skilled eyes of a Dermatologist to really diagnose and recommend proper treatment.
My guess is that the pediatrician/nurse will recommend Benadryl/Zyrtec to see if it is an allergic reaction. Pollen counts are so high, she could have come into contact with something foreign and simply be having a reaction.
The other possibility is some kind of contact dermatitis which can be caused by just about anything - bacterial, fungal, viral - or eczema or even heat rash.
I think you're doing the right thing by calling her pediatrician in the morning. I'd wait until then to purchase anything because of her age and not knowing a recommended dose or proper diagnosis.