Chihuahua/Corgi in pain

Question
My Chihuahua/Corgi mix is 3 years and 7 months old. Five days ago he started to act abnormal. He wasn't as playful as he usually is, showed no interest in his dog food, and did not want to jump onto furniture or use his doggy stairs. He even cried when I picked him up. I took him to the vet on day 2. They did an exam and could not find anything wrong. The vet told me to feed him turkey and rice and see if he gets better. He was still yelling in pain and avoiding furniture so I took him back on day 4 (yesterday). The vet checked him again and found no pain sites. She expressed his anal sacs and they were full so she said she thinks that caused his discomfort. He was curled around licking after they were expressed so she ruled out back pain. Last night he was grunting in pain while in bed and yelped this morning when I picked him off the bed to put him on the floor. He has an appointment at 5 this afternoon and the vet wants to give him pain medication but I'd like to find what's causing him pain, not just give him meds to suppress it.
What could this be and what tests should be done for him (blood, xrays, etc).  He goes in annually for his physical and is up to date with all of his vaccines. He's eating, drinking, has normal stool, and is
not vomitting. Seems very comfortable while he sleeps but wakes up and seems like he's in pain but we cannot localize it. He does not limp.  

Answer
Pain that is not localized could be nerve pain. Dogs and cats both can get a neuralgia in the area around the tail head on top. If he flicks his skin around the area where you would scratch him just at the top of his tail, kind of like a flinch or quiver, then he could have nerve pain there. Another place they get pain is in the sacrum.

I don't care if he turned and licked, if she hasn't checked his sacrum, then he could have pain there. It is very common in little dogs that jump up and off of furniture.And once his anal glands were expressed, his back pain was the least of his concerns at that moment.

The very fact that he is avoiding stairs and furniture are a big indicator of sacral pain. That would be the area where his hips attach to his pelvis.

An xray might show something, more likely not unless he knocked his pelvis out of joint. Instead of just a pain shot, she should put him on some Prednisone for about a week and see if that takes down the inflammation in the area that hurts. 9- 1 it's his lower back. If he is long like a corgi he has an even greater chance of injuring his sacrum but we see it in all kinds of small dogs.

These are just some ideas for you- I am not diagnosing that that is the issue here, but that is where I would start.