Honestly there could a million and one things this is what comes to my mind, fleas, or a skin reaction to a new food started. Or she or he could have allergies also I would take the dog to the vet myself to know for sure, better safe than sorry.
Common culprit for dog itching would be ticks or mites. If there is no visible sign of these parasites, then it is most likely mange. See a veterinarian right away.
She could have allergies, Or it is possible she could have the mange, or flea allergy dermatitis. You can treat the latter with a product called Revolution. It is for heartworm prevention, fleas, and mites that cause the mange. Revolution is available from your vet and should only be given if she tests heartworm negative.
There are a lot of things, like flees, ticks, or simply dried out irritated skin. Try giving your dog a bath, scrubbing deep and rinsing your dog's fur thoroughly.