Hello, I'm Dr. Zoe. It sounds like we have a few issues going on with Harley Ann.
First, with the trembling and lethargy this is concerning. It is unclear based on what has so far been discussed how long these symptoms have been occurring (just today or off-and-on); but sounds like it has been recurring. As Dr. Peter mentioned trembling can be a sign of fever or pain. It can also be a sign of an endocrine condition called Addition's disease, but I really would expect to have other signs start to develop such as diarrhea, although if she only has corticosteroid deficiency then the trembling alone is possible. I'm also not certain if this could be related to her other issues...so for now let's talk about the two other issues you mentioned.
Foul smelling breath? This could indicate significant dental
disease. This breed is notorious for having significant periodontal disease
and abscessed teeth. If you see heavy calculus on her teeth and/or pus at her gumline then she has bad dental disease and this could be the source of the bad breath
. If you are confident that this cannot be the case (her teeth are great or she just had them cleaned by your vet recently), then the bad breath may be an indication of an internal illness such as kidney disease or diabetes
Chewing at hind limbs/flanks? I'm not sure this is related to her other issues. If she has skin lesions then I worry that this reflects a skin condition. But suspect since you didn't mention lesions that this is not the case. Fleas is certainly a good thought as it does cause spontaneous chewing/biting at these areas. But in a dog of this size you should be able to fan through her fur around her neck and base of the tail and either see the fleas or flea dirt (black dirt that if you put on a piece of white paper and then ran a wet finger through it would smear red...digested blood). What could perhaps link back to trembling is if she is experiencing lower back or sacral pain. Then it is possible for her to bite at these areas. Your vet may be able to detect this with a physical exam.
Stool change? Smaller than normal stool likely reflects her reduced intake, and dehydration. ARe her gums tacky to the touch or dry which would correlate with dehydration? What is good is that the stool is formed. Was it of normal color (not black or lacking color)? I assume so since this wasn't mentioned.
Overall, I'm quite concerned. If she was my patient I would strongly recommend some screening lab work, if this hasn't been done in the past 2 weeks. THis would include a CBC, cHemistry profile and urinalysis which would screen for diabetes, electrolytes (if off or high potassium then Addison's disease comes back into question), liver/kidney, etc. It's really a starting point. As well your vet can take a look at her mouth to determine if the source of odor could be a tooth (or teeth) and check her skin to determine if there is an underlying problem such as infection (bacteria, yeast, mites).
At this point I need your input to let me know your questions.