You raise several important issues.
all, while kidney disease may make animals (and people) feel lousy (weakness, dehydration), in and of
it itself it is not a painful disease. Some sequelae (consequences) of
it can be painful such as stomach ulcers, oral ulcers, movement of
uroliths (stones) or blockage of
urinary pathways by such stones. It is not clear from your post why the dog is taking pain relief medication in the first place.