Hi and welcome!
it sounds very much like fungal, possibly ringworm. Please have your piggy seen and diagnosed by a vet. Do not home treat on spec if possible, as that can go horribly wrong. You will need to treat any companions, too, as fungal is infectious. In the case of ringworm, it can jump species, so you need to be very careful to not infect yourself or any other pets and persons living in the same household.
If your piggies have come down with it within two weeks of getting them from a shop, you should be able to reclaim any vet cost from them.
https://www.theguineapigforum.co.uk/threads/ringworm-hygiene-and-pictures.115402/
As we have members from all over the world, we find it very helpful if you please added your country, state or (for the UK) your county to your details, so we can always tailor any advice or recommendations to what is available or possible for you straight away. Click on your username on the top bar, then go to personal details and scroll down to location.
We have got a piggy savvy vet locator on the top bar for our UK members, but we may able to provide links for other countries.
