I agree with all the above. My oldest at 15 has spine issues and has been on rimadyl low dose for about a year with great results, he had pain before, but has had a very good year. Eddie (11) is on thyroid meds, he was a cranky-pants a couple of years ago and after thyroid meds, is a happier dog.
A vet visit is in order to figure out what has caused his change, 11 is still pretty young.