My dog just started too. My vet said that it's seasonal allergies. Look inside your dogs ears. If they're dirty, get ear wash and clean them out with cotton balls. Check between your dog's toes for discoloration on the fur. If the fur is brown, reddish brown, or pink, she could have a yeast infection between her toes. That is a pretty common sign of allergies. Also check to see if she has little red bumps on her skin. That's another sign. Petsmart and Petco sell medicated shampoos. This is the only time in a dog's life that you should wash him/her once a week. Wash her feet and arm pits twice a week. It has made a huge difference for my dog. I refused to pay $1,000 for the medication the vet tried to sell me.
Allergies can be airborn or imbedded in toys, the carpet, pillows, dog beds, etc. I recommend washing everything you possibly can or spray a disinfectant spray to try to kill germs.