If she has a thyroid problem and the medication isn't controlling it properly, that could cause her weight loss. Also, if she hasn't been eating much or at all, that would cause the weight loss as well. I know that some cats on Tapazole/methimazole have problems with vomiting and appetite loss. I would definitely listen to your vet about taking her in if she doesn't start eating very soon. Cats can get very sick if they don't eat.