Worried about my cat's health?...?

nine years ago we got a dog, now 9 years old. my cat is about 16 and has been in my family since she was a kitten. i don't know how long this has been going on for since we've only just noticed, but every time we come back from holiday (the dog with us), when we return our cat is beautifully healthy and not skinny in that "old cat" way. as the weeks go past, she gradually gets thinner and thinner until she's just a back to a bag of bones. the neighbours who feed her when we're away tell us she eats wonderfully, but when the dog is around she eats barely at all. we think the dog is stressing her out? she is definitely the boss in the relationship between her and the dog and makes it very clear through whacking her round the head sometimes. what can i do? is my dog stressing out my cat? what can i do to help her improve her health and to eat more?...