Has anything changed in her diet? If she has always eaten the same thing and only recently started itching, I would look at other possibilities. How about shampoos? Are you or your groomer using a different shampoo? Could she not be getting rinsed well enough? That is one of the biggest reasons for itching -- it is sometimes difficult to get all of the product out of our pups' hair. Rinse, rinse, and rinse some more!
Vets say the itching comes from one of six causes:
Environmental
Nutritional
Parasitic
Allergic
Neurogenic
Infectious
Here's a link to run you through each of those possibilities:
Cat and Dog Skin Problems | Itch-And-Scratch-Bite-And-Lick! | petMD