This is pretty common, especially in these little dogs.
I would talk with the vet about a different low allergy diet, one that will have the least reaction. Rabbit and potato work well and you can get it at your local vet's office or maybe even pet smart.
You should also get her on a flea preventative if she is not already. Even if you don't see fleas, it only takes one to get the cycle started. Some dogs are highly allergic to flea bites.
I would also use a prescribed dose of benadryl, especially during allergy season. My vet recommends 1 mg per lb of body weight. Talk with your vet about this.
Keep in mind that dogs can have allergies to pollen, dust and other household allergens so you should evaluate your home and try to make it as allergy free as you possibly can for her.
While you are there, you should also speak with the vet about a supplement that has omega three fatty acids. Good skin starts from the inside out and this can be very beneficial.
Let me know if you still have specific questions about this.