I suggest getting her to the vet or emergency vet as soon as possible. Take her to a different vet if you can. Cats benefit from a medication called methocarbamol which is a muscle relaxant that can be given through an IV. She should also be receiving fluids through an IV as well.
They can give valium if necessary as well as an appetite stimulant.
Your cat must be treated and have more intervention then what this vet has offered you. She needs to be force fed every 3-4 hours using a highly palatable yet thin food that can be given through a syringe. Shaving the area where the flea med was applied can also help if this is within the first 12 hours.
Talk to your vet about hospitalizing her on IV fluids for forced feeding and Robaxin or other medication to control muscle tremors.