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It could be fungal or the beginning of mange mites, but you will need a vet diagnosis, as treating on spec with too low dosed shop products can make things much worse instead of better. You also need to have all guinea pigs living together seen and treated as both conditions are transmittable. Some forms of fungal (ringworm) can also spread to humans and other pets.
We have got a UK piggy savvy vets locator on the top bar, but can also provide recommendations for other countries if wished.
Could you please add your country, state or (for the UK) your county to your details, so we can help you better? We have members from all over the world, and medical as well as supply brands, climate, access to vets and rescues etc. can vary enormously. Click on your username on the top bar, then go to personal details and scroll down to location. Thanks!