Hello everyone, situation ? My daughter is 6 months old and literally …

Hello everyone, situation ? My daughter is 6 months old and literally an hour ago there was nothing, but now there's a rash, it's nowhere else except on the back of her head and hand from elbow to wrist. What could it be? I have an appointment with the pediatrician tomorrow, but suddenly anyone can tell me, is it eczema? Or allergy to the powder? But we use the powder for a long time and everything was fine.