Hello, XXXXX XXX XXXXX
What you are describing sounds like Feline Hyperesthesia (FHS). It is also referred to as the rolling skin disease. Some Cats feel like they are being bitten by fleas, when they are not. At times in some cases you can actually see the skin moving like it's "rolling". There are a lot of different symptoms for this, such as sudden bazaar behaviors, insane aggression towards their own the tail, seemingly compulsive grooming, hiding, running around like they are crazy, and extreme sensitivity to the back/tail area, staring into space like they are day dreaming or what seems like they see something there when there isn't anything there, among other things.
It is believed to be an obsessive compulsive disorder.
You can read about it in detail here:
http://www.petplace.com/cats/feline-hyperesthesia/page1.aspx
The most common treatment for this is to just eliminate stress as much as possible for the cat. Improving his/her environment with toys that encourage aerobic exercise such as laser mice and feather wands, play time with the owner, frequent feeding so the cat does not stress over food, window perches with bird feeders outside, leaving the TV on while you are gone or a radio on low. Pretty much any thing that enriches the cats life at home.
You can also try a calming pheromone called Feliway. This is a cat pheromone that induces calm happy feelings in cats. It helps to eliminate stress, anxiety, aggression, territorial behaviors, etc. You can find it at Pet Smart.
There are drugs available as well to help. These are serotonin-enhancing medications like paxil and prozac.
Because your vet didn't seem to be sensitive to the issue with her back end area, maybe a more empathetic vet would be a good idea.
If you need any other help or assistance with this please let me know.
Sincerely, XXXXX
Expert in cat behavior and health
Expert in cat medical and behavior issues.