Fleas can cause anaemia, and can even kill a cat or dog if untreated. Anaemia is a condition of overall weakness that can contribute to a whole host of other health problems that would otherwise be less significant or not present at all. This includes problems that on the surface seem completely unrelated to fleas.
If she likes canned food, let her eat as much of it as she wants. Treating dehydration I might even take away her dry food altogether, providing she is enthusiastic about the wet food to get all her calories that way.
It is good that you are keeping in close touch with your vet, because if things do not go the way they expected, your vet may want to change medications or adjust dosages. Work on those fleas, have you considered shaving her to help with that? Hope you will soon start to see some improvement.