My Granddaughter Went For A Plant Identification Walk With My

My granddaughter went for a plant identification walk with my husband last evening. This morning, she was sent home from preschool because
she she had small blotches, mostly on her face. She had no other symptoms and her doctor said that she has hives, and that they would clear up without treatment. I'm wondering what might have caused the problem. Could she be allergic to some local plants? W#e live in Northeastern Pennsylvania.