You raise several important issues.
First of 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.