I'm not sure what your Vet has told you, so please excuse any information that you already are aware of. Just in case this is related to dry skin, you may want to supplement your dog's diet with Omega n3 fatty acids. You can do this using fish oil capsules.
Since the Benadryl relieves the itching, and based on other things you have mentioned, it seems your dog may have an allergy. You can read up on allergies here:
You can read up on allergies here:
The problem is trying to determine just what she may be allergic to. Since she was fine for 6 months, this signals a seasonal allergy. Of course, that means we are looking for something that is around in the summer and something that is around now, but wasn't present in between. For example, I had a client whose dog was allergic to an air freshener add on in the heating and cooling system, so the dog had flare ups of allergies during those times that the A/C was used or the heat was on, but not in the spring and fall when the windows were open. This is an example only.
So in one way, you will need to put your detective skills to work to see if you can remember any specific thing that may have been used during these times or fed during these times that is missing at other times. Think about things such as cleaners, things you may spray on areas that she lays on. With an allergy, one exposure can cause itching for awhile. With allergies, benadryl does help control the itching, so you will want to use is as recommended.
The problem with allergies is that a dog can develop secondary skin infections such as staph or yeast dermatitis which require antibiotics and/or shampoos or creams to help clear them up. 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 though it does not cure the allergy. 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 but can affect the whole body. Selsun Blue Shampoo can help with Yeast dermatitis as can miconazole cream for areas such as the paws and armpits. When shampooing, lather and leave on 15 minutes before rinsing. These shampoos are not meant to be a cure just a relief.
You will want to have her examined by your vet and discuss this with him. You may also want to see a dermatologist to determine the exact cause for the allergy or use the elimination method described on the website I mentioned.
I hope this has proved helpful to you.