It sounds as though she most likely has mites, possibly mange mites. This is not uncommon in guinea pigs, unfortunately, but is easily treatable.
She will need to be examined by a veterinarian who is experienced with rodents. If they do diagnose her as having mites, she will need to be treated for them. This is typically done using Ivomec, or Ivermectin, although some veterinarians will prefer to use a different medication.
As I mentioned above, my advice would be to have her seen by a vet, so that they can diagnose, or rule out, any possible illness she may have, and determine an appropriate course of treatment.
Hope this helps!
I am NOT, nor do I claim to be, a Veterinarian.
Always check with your vet for any symptoms your pet exhibits.
Please press Accept if you find my answer helpful.