Take your kitty back to the vet and have them treat for parasites. I have a siamese cat and when he was kitten he was extremely ill for about three months. We tried everything. We finally took him to a vet who treated him for parasites, not just the regular deworming treatment. He gave him medication that would get rid of any type of parasite that could be living in his body. After that and some antibiotics he was fine and is a very happy kitty now. Just keep trying It will be okay.