My lab is acting get strange need help please

My 6 year old yellow lab is yelping in pain once again. About 2 weeks ago, he started randomly yelping in pain. He would be laying down and then all of a sudden, yelp. I tried to very lightly pet him, and he yelped when I got near his lower, back, upper hind legs.

*Became very lethargic and immobile, wouldn't even get up to eat a treat on the floor 2 feet away, which is very worrisome for him to display this. We brought him to the vet twice. X ray showed nothing wrong with skeleton.* She prescribed tramadol and an nsaid , something profen, carprofen maybe.

*He got better, almost 100% in about 4,5 days, then today, after being well for a solid 6 days, I went to pet him and yelped when I got near his FRONT legs.. (upper) I'm extremely worried, he is so loved by us. What could be wrong???

I built him a long ramp to get down the 3 deck stairs, where he potties. He's not using the stairs in the house, sometimes sneaks up on a couch, but I try to keep him resting

No limping by the way.

Thank you so much.

I wouldn't dare make recommendations other than see a vet. Could be spinal.
Pain often shows up as lethargy in Labs. I've seen arthritic Labs change from subdued couch potatoes to normal crazy dogs when given proper medication for arthritis, especially when combined with a weight loss regime. Overweight Labs are much more prone to getting into muscular/skeletal distress than slender ones.

I would prob suggest an MRI to rule out anything spinal cord related.