The lameness issue may be unrelated to the itching. The itching does sound like either an environmental or food allergy and should be treated by your veterinarian. The rawness and bleeding means that she probably requires an antibiotic for the secondary pyoderma (bacterial skin infection) that she caused as a result of self-inflicted skin trauma from biting and scratching. She may also require a steroid to calm the inflammation and antihistamines. Veterinary dermatologists are now using a combination of two or more common antihistamines such as: Benadryl, Claritin and Zyrtec for canine patients since the combination seems to help just a bit more than a single antihistamine alone. However please discuss antihistamine use with your veterinarian before instituting. There is another prescription anti-itch product called Apoquel, which will help but it has side effects and Apoquel should be discussed with your veterinarian to see if it is right for your dog.
Additionally, If your dog has environmental allergies, allergy testing and subsequent allergy shots (to desensitize your dog to the allergens) can be given. The allergy shots can give added relief without the use of medications. Currently there are no reliable allergy shots for food allergies. The only treatment for food allergy at this time is avoidance of the offending food.
It is unlikely that the Frontline is causing an allergic reaction since you placed it on your dog 2 weeks ago. If allergic to Frontline the symptoms would have been apparent within 4 days, and usually the symptoms occur within minutes to hours.
Lastly, the lameness could be due to anything, such as: a soft tissue injury (sprain or strain), an insect bite, degenerative joint disease (arthritis), Lyme or other tick borne disease, a splinter or other foreign body, a broken claw and many more. To be safe your dog probably should be tested for Tick borne diseases. This is done with a blood test.
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