My name is***** and I am sorry to hear that your dog is not feeling well this evening.
Based on your history, I would wonder if he has back pain again? What other behavior changes are you noticing? Is he jumping on and off furniture normally? Is he lifting his head to look up at you or his he holding his head down and not moving it much?
My next thought would be abdominal pain. Usually, though, with abdominal pain, dogs will stop eating and become very lethargic. They may also have vomiting or diarrhea. Possible organs that would be involved with abdominal pain include the pancreas and the gall bladder.
Cancer is pretty low on my list of possible causes of his pain. Dogs are rarely painful from cancer and the first sign we usually see is not eating and drinking.
For tonight, do you have any pain medication left over from his back surgeries like Tramadol or Rimadyl? I would recommend giving him the prescribed dose to help him be more comfortable while you are waiting for his appointment tomorrow.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001