It sounds like she likely has a yeast or staph dermatitis. These skin conditions are generally seen with allergies. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching. Yeast likes areas such as between toes, armpits and ears. Selsun Blue Shampoo can help with Yeast dermatitis. When shampooing, lather and leave on 15 minutes before rinsing. These shampoos are not meant to be a cure, just a relief until your pet can see a Vet. Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours for itching due to allergies.
You can read about allergies here: http://www.vcahospitals.com/main/pet-health-information/article/animal-health/allergy-general-in-dogs/428
Here is a site that goes over itching in dogs.
Now if a dog is scratching for any reason, it can damage the skin's immune system and lead to the staph and y east normally on the skin to multiply unchecked by the lowered immune system. So the allergen may have been something you have only fed occasionally which started the itching and now the skin dermatitis is causing the itchiness and continuing the cycle.
I would likely go ahead and start using benadryl. I'd also switch her to a limited ingredient dog food such as venison and potato. You will need to have the dermatitis treated at the same time. It can take 6 weeks or longer to see improvement after the allergen is eliminated but as long as the dermatitis is not treated, you won't see improvement. Thus both the allergy and dermattis need to be treated together to achieve the desired result.
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