I would stick to what your vet recommends for flea control. You do have to be extremely careful with over the counter flea product or home remedies due to possible toxins. Dogs don't metabolize certain things like humans do. Always consult with a veterinarian before trying anything. The next time she's at the vet I would bring up the issue of the shaking. Being that she's an old timer it could just be due to her age, but wouldn't hurt to bring it up. As long as she's eating and drinking normally and not having any diarrhea or vomiting she should be okay.