Hi. My best friend had a Birman who lived to be about 19 years old and has recently passed away. An eight year old cat should NOT be fed on kitten food, so you had best stop that! It is way too high in protein for her so a food for a senior cat would be more appropriate. If she is thin, it could be a sign or worms or something else, so you are very right in taking her to a vet for a thorough check up and they will be able to advise you better on nutrition for her. She is obviously telling you that something is not right, or is just a very needful cat. Good luck and hope you get her sorted out!