How do I get my dog to stop scratching her back on everything?

She does a bear scratch on her back against anything she can find.

She gets in a frenzy, starts moaning and getting excited because it feels so good, I guess.

We took her to the vet and he said there are no fleas.

I don’t know what a hot spot looks like but the base of her tail, where she likes to scratch, looks a little rubbed down but not too bad.
The Vet didn’t know why she was scratching so much. They ran the full gambit.