Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry that Bella is not feeling well....
Most likely Bella has suffered an injury to a disc in her back. This is very common in jack russels because of their conformation. Since Bella does not have any digestive issues, this is the most likely cause for this pain that is beong shown by not wanting to move and her feeling cranky when your other dog comes near. Since this is deep within the vertebrae, she will not show pain when you touch or palpate these areas. This can happen when Bella jumps up or down on something or just moves the wrong way. The best thing you can do for now as long as Bella is eating well now, is to give her aspirin at 5 mg per pound every 12 hrs. This will decrease the inflammation that is causing this pain and start to stabilize this disc. Do not allow her to jump on or off anything or even climb stairs to allow this disc to stabilize. It is best to have your vet examine Bella to be sure that this is the case and that this treatment is appropriate.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, XXXXX XXXXX