Generally--GENERALLY--but not always:

flea allergies: base of tail

food allergies: ear infections (American cocker spaniels are notorious for this since they are both food-sensitive and ear-infection prone)

inhalant allergies: skin infections

Food and inhalants are probably your most likely bets. Or possibly a case of an isolated irritant (burr under her collar, etc.) Our first dog used to chew holes in his legs because of his inhalant allergies, but he did it chronically, every single year.

Whatever it is, I hope it clears up fast.