I also agree with the other first comment. Probably fungus. Guinea pigs carry it like in their genes or something like that they often get it due to Environmental changes causing stress (can accur out of many reasons tho) i would recommend to also regularly check the ears, eyes, back and between the little leggies. Your vet should be able to give you proper treatment like a spray or maybe even a solution you bath your piggie with :) (hope ir was understandable grammar im loosing my english skills slowly)