This could be many things :) She could have an allergy to flea bites...(VERY common) ..if you are not using a monthly flea preventative, talk to your vet about Frontline Plus or Advantage for her :) It could be an allergic reaction to a plant in your yard or a chemical like a carpet refresher or something of the like :)
Try using a nice oatmeal shampoo on her. Arm and Hammer makes a really nice one and you can safely give her some plain benedryl to help relieve the itch. 1mg per pound of body wieght is the recommended dosage..ever 8 hours as needed.
If it gets worse, please consult your vet regarding this problem :) Good luck and hope she gets some relief soon. :)
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