Shih Tzu sick?

Question
Hello! We have a 2 y/o male, neutered shih tzu in good health. Beginning this afternoon, we noticed a change in behavior. He doesn't appear to want to go on walks (while walking, he'll sit down on the sidewalk as if he doesn't want to, he loves his walks!), he didn't touch his dinner, and now appears to not want to, or can't, jump up on the couch, or anything else, overall, he's frighteningly not himself. He is still urinating, and has had 2 bowel movements today, (normal), but his demeanor tells us something is wrong...I know you cannot diagnose my baby, but can you please give us a laundry list of ideas?

Answer
Hi - Ok, he must be in some kind of discomfort. ( as you already know).
Manipulate all his limbs.  Work each leg up and down slowly - Also push your hand under his paw pads and place a small amount of pressure.  Does he have any pain or discomfort when you manipulate him?
Look in his mouth ( if he will allow) - Do you see any loose teeth or red areas?
Are his gums nice and pink or are they gray in color?

Let me know what you find?

Marie