Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear about Robbie's condition. I know this is very stressful for you and for Robbie.
Most likely Robbie has suffered an injury to a disc in his back. This is very common in older Corgis because of their conformation. Since Robbie does not have any digestive issues, this is the most likely cause for his pain. Since this is deep within the vertebrae, he will not show pain when you touch or palpate these areas. This can happen when he jumps up or down on something or just moves the wrong way. The best thing you can do for now as long as Robbie is eating well now, is to give him aspirin at 5 mg per pound every 12 hrs. This will decrease the inflammation that is causing his pain and start to stabilize this disc. Do not allow him to jump on or off anything or even climb stairs to allow this disc to stabilize. It is best to have your vet examine Robbie 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, Dr. Loretta