What flea meds are you using? . What medicated shampoo did the vet give you?
I agree with both suggestions.Yikes why on earth would the vet allow you to wash your dog once a week with flea shampoo? Flea shampoo, even if its "all natural" is not for once a week use. Your flea meds should take care of the problem.
Did the vet express her anal glands?