Hi there
There's a possibility this could be mites or a fungal infection such as ringworm (there are others) if it is, it's highly contagious.
Ringworm is very common in shop bought piggies in the first few weeks - it's an opportunist that usually strikes when the Guineas are stressed with a low immune system.
My Tiggy had a dry scabby patch on her back, that I thought was a scratch mark to begin with, then it grew very quickly into a bald patch & she had ringworm, she had a very big sore patch on her back, the vet prescribed some Malaseb anti fungal shampoo, but other members have had a quicker healing process with itrafungal oral medication (prescription only&usually expensive).
All 4 of my girls got ringworm & needed treatment, but they've all recovered now.
If you bought your Guinea in the UK you should be able to claim your vet treatment costs if it's a pet store.
I think a while ago it was someone from the USA that had to surrender their piggy back for treatment at the store&left all details. The store then contacted her when piggy had recovered. (personally I'd prefer not to give back the piggy as I would be afraid I wouldn't get her back, but it worked out fine for the member on here)
I'm not sure where you stand if it's a breeder.
Some vets in the UK do pet plans as suggested above.
Please have her seen as soon as possible by a Guinea savvy vet
(recommended UK Guinea vets link on menu bar on here).
For the USA check on the Guinealynx website for recommendations
If it is fungal, time is of the essence... It will also get worse before it gets better
There's a few links/stickies on here that are good for ringworm advice.
Ringworm: Hygiene And Pictures
Please see a vet for a proper diagnosis & treatment though.
It is a bank holiday on Monday here in the UK so an appointment today would probably be easier & a lot cheaper if you're based here, also vets for pets branches don't charge generally for weekend appointments.
Let us know how u get on, good luck & healing vibes to your little one.

Xx