Does she have any allergy? It could be she might have hives, some unknown allergy or could be chicken pox. Most of the times it can be due to hot weather this happened. You might want to go back to the same doctor to get it check. Alternatively to go poly and they will advise further. I dont suggest hospital because it is expensive to do walk in.
Meanwhile sponge down with cold towel, wear thin clothes, fan/AC on. Use Sebamed/Cetaphil product, then put on the prescribe cream and give medication.