It could be: Yeast (lotrimin), ringworm (common from playing outside or pets, petting zoos, etc. RX cream), excema (RX)...With eldest being 20 and youngest 9-I've run the gamut. Keep supplies on hand. One son was so sensitive had to use cloth diapers with vinegar rinses and hats caused rashes on forehead, and one gets ringowrm due to playing in dirt and a daughter has excema and a daughter has allergy to metal and esily yeasty (especially with antibiotics). Usually minor-keep a supply of various-you should know within days if it is responding. Also-various delivery agents are irritating to my skin-may need to switch from cream to gel, for instance.
Congrats-we did infertility for years and got our four!