Help, my dog has been diagnosed as anorexic

My Standard Poodle has been diagnosed with anorexia. This problem started a few months ago and reached crisis point a few weeks ago. We have been doing quite well with me cooking her food and gradually adding a little dried food. What she eats one day, she might turn up her nose at another day. I think I have more varieties of food in my house than the pet shop does. If she won't eat we do not make a fuss and try again later. There is no plausible reason why this has happened as I have two other dogs and they all get on well together. I was wondering if anyone else has come across this problem with their dog. Her body went into starvation mode and she has hardly been passing any motions but today she has chronic diarrhoea which has set us back. We have done all that we possibly can but any other suggestions would be most gratefully received.
Judith Stephens, Mrs