Thanks for your help.
She has normal stools, no diarrhea, and no vomiting. I gave her some deworming medication about 3 months ago thinking that it might be worms. I will try changing to a different dog food that is higher in carbs and will get some probiotic supplements. She is an outdoor dog along with her brother. He is a strong healthy male who is almost twice her size and is fed on the same diet - it is odd that two dogs could be so different in their nutrition requirements and be from the same parents.
What blood work would need to be done to rule out heart disease, thyroid problems, pancreas or kidneys problems? Do you know the usual cost for these tests?