Hi there,
By far the most common cause of these signs in a guinea pig is an extrnal parasite - usually a mite but also occasionally a louse infestation. These often come in on bathces of bedding such as hay or straw, or they can be present in small numbers and then cause a problem if the guinea pig is poorly for another reason.
Therefore it is important to assess whether your pig is otherwise well, eating, pooing, moving around etc. Then I would recommend vet treatment for this problem as it sounds severe. There are pet shop meds that you can buy for mites, but these are not always powerful enough, and also your pig may need antibiotics or anti-inflammatories for theskin too, which your vet can give.
Clean out all of the cage and throw away the batches of bedding that you hvae, just in case this is the source of the problem.
I hope this helps,
Jen
Veterinarian
UK small animal vet and acupuncturist with an interest in rabbits,geriatric patients and surgery.