How Can I Get My Dog With Flea Allergies To Stop Itching Even After I’ve Gotten Rid Of All The Fleas?

I”ve taken my dog to the vet multiple times for itching and he said she has flea allergies. Even if only one flea gets on her, her entire body can become irritated. Last time, I spent $ 250 at the vet! But she doesn’t have any allergies besides fleas. Well, went on vacation and left her with the inlaws, came home, and there were a few fleas on her. We got rid of all the fleas with frontline and have been checking daliy. They have been gone for 4 days but she is still itching like crazy! Scratching all night and her back is sooo sensitive. What can I do to get her feeling better without going back to the vet??? (The itch relieving shampoo hasn’t worked.)