Sounds like Allergies to me...
there are quite a few members whose dogs have allergies to one thing or another...you may want to look into switching her food...
Use something with a simple ingredient list no fillers, dyes, by products....California Natural is made for dogs with allergies (there are others too i just don't know them off the top of my head) You could also try boosting her immune system a bit with some supplements, vitamin C, Echinacea and of course garlic...
and of course take her to a vet....excessive Itching, scratching, gnawing or even "butt scooting" (you'll know that if you see it) is definately not normal.
Trudy's Mom (aka Julie)
Trudy 10/30/01, Brindle, Black Mask, white paws, Natural ears