Gnawing at the base of the tail does have a strong link to a flea allergic dermatitis (a reaction to flea saliva), so as you know your cat had fleas, this would be the top of the list.
If a dog has an allergy to fleas, they will over-react to even one bite and can become excessively itchy and uncomfortable. It's great that you have already given her a flea treatment. It may be that the fleas have gone but the irritation remains.
It is essential that you use an effective flea treatment on both the cat and dog (and any other cats or dogs they are in contact with). Not all flea products are created equal and some are not very effective. I have most success with a brand called Advocate.
Simply applying the flea treatment is only half the battle as 95% of the flea burden (eggs, larvae etc) live off the animals and in the house, so without treating the house, the problem will continue. All bedding should be hot washed at over 60'c, the floors need to be hoovered and it would be worth getting a flea spray e.g. RIP Fleas or Indorex.
It's good that the anti-histamines have helped a little. You can also bathe the tail base skin in a dilute solution of chlorhexidine (hibiscrub) or warm salty water to sooth the skin and prevent infection. This should be done twice a day.
DO NOT allow your dog to continue chewing/gnawing as this is exacerbating the issue. A buster collar / elizabethan collar may be necessary. Continued chewing will result in worse skin irritation and potentially a skin infection.
If things do not settle with the above measures, your pooch may need a stronger anti-itch medication from the vet to break the itch-scratch cycle they are now in, so it's good you have an appointment on Wednesday.
A final note, an irritated tail base can also occur with blocked anal glands. If you feel comfortable expressing the glands at home, do so in the bath tub with someone else holding your dog firmly. If you are unsure, this is something your vet can check for you.