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It could be hip pain, tail pain, rectal pain, or urinary pain...
Hip pain will manifest with slowed ability to sit or get up...reluctance to go up stairs and sometimes walking like they are on egg shells.
The main thing is when a dog actually shows pain, especially a lab, the pain is pretty bad. I would recommend a trip to the vet so that they can identify the problem and start medication immediately. Chronic pain is hard to deal with, but we have pretty good medications. If it is hip, make sure you start vitamin and glucosamine support.
Good luck and FYI...wt loss helps alot, even if you get him on the thinner side.