I noticed tonight that my cat has this bumpy (small bumps) but bright red it seems all the way up her spine starting with a huge cluster at her tail. At first I was thinking - she ran a bit behind in her flea medicine and she was scratching quite a bit - but that's been taken care of - no flea dirt no noticeable little fleas - nothing - but the rash has appeared rather over night it seems - any ideas??

By taken care of, you mean you have applied a flea med?? When? If they are broken out from a flea allergy, it will take a few days befoe the rash will disappear.

Many cats are allergic to fleas and one bite can cause a reaction of little red bumps. I'd go over her with a flea comb to be sure there are no fleas on her.

Or take her to the vet and have him look at her.

Sounds like you should see the vet for an antihistamine.