It sounds like this could be pain related and I have to agree with Cindy here - vet visit.
If the dog acted great 1 week and the following week (more than a few days) the dog isn't himself - this indicates a problem. What kind of problem is nearly impossible to guess but given the dog's age, Cindy's guess of possible arthritis is a good one and would definitely be something to mention to the vet.
What specific differences in behaviour are you noticing?
(i.e. less playing, less eating, less affectionate etc.)
Answers to these questions might suggest something to one of the members.