Thanks for the additional info.
The first concern I have is possible Addison's disease, mainly because Standard Poodles are genetically predisposed to the disease and because it often affects the GI system. Here is a website with more info: http://www.veterinarypartner.com/Content.plx?P=A&A=608
-- see what you think.
But I have a few patients that are similar to Emma--poor eaters, skinny, active, normal bloodwork. Their physical exams are all normal, except that they are very lean/skinny dogs. They seem to maintain their weight, just not gain any. I think sometimes they are just like those skinny kids some people have. My boys have always been "sturdy" (not overweight, but certainly not skinny!), but I look at some of their friends who have have skinny legs and you can count their ribs. I don't think there is anything wrong with them, just genetics and they are active.
If Emma hasn't had a broad-spectrum dewormer (even with a negative stool sample), I would ask your vet about prescribing Panacur or Drontal plus. Parasite eggs can be intermittently shed and some (like whipworms) can be difficult to find, so deworming would be worthwhile and not cause Emma any harm.
I would make sure that the bloodwork included electrolytes, specifically sodium (Na) and potassium (K), and that the ratio of Na/K was over 27 (if low, then Addison's is more likely).
If you think, after reading the article, that Addison's is possible, you could also have a baseline cortisol checked. This isn't diagnostic for Addison's, and if it is low she would still need to be tested, but it won't be as expensive as the ACTH stim if money is a concern.
Since the problem seemed to occur after her adult teeth erupted, I would make sure that all of her permanent teeth are accounted for. If one didn't erupt it could cause problems under the gum and cause chronic mouth pain.
I would also try (again) different foods. Mix in some chicken broth with her dry food, mix in some canned dog food, try a food with limited ingredients in case she is having some stomach upset from an ingredient. If you think she is a little nauseous (smacking her lips), you could try giving her 10 mg of Pepcid twice a day.
Hope that helps!