Thank you .
The first thing I would double check is the thyroid if not tested then I would do a full thyroid panel. I say this because some of the symptoms we can see with low thyroid are exercise intolerance and panting.
I mention the neurologist because it is possible that this panting and being unable to continue the walks is related to pain and often this pain is in the spine. This could be due to disc degeneration, compression of discs causing compression of the nerves, tumors on the spine, arthritis in the spine, Spondylosis
Most spinal problems are not able to be seen via x ray and more advanced imaging such as and MRI are needed.
If this is happening more at night then there is a slight chance this could be associated with canine cognitive dysfunction though I am not sure this would explain the walking issue. Link
As for the Jerky treats these normally result in a condition called Fanconi syndrome which causes acute kidney failure. The problem is that the exact cause of the problem with the treats have not been identified which is why they are still on the market.As for the sulfaclozine the most common problems we see are intestinal upset, allergic reaction such as itching and trouble breathing read more about this here
The symptoms that Jelly Bean is showing would not be associated with what we normally see as a result of the treats and I think something else is going on. I would check on the thyroid if this was normal I would then consult with a neurologist. The next option after a neurologist would be an internal medicine specialist.
I hope this helps give you some additional options to discuss with your vet.Please reply back if you have additional questions and I will be happy to continue.