New problem-left side swelling

My dog is swelled up on his left side under his rib cage. The Vet took an x-ray and came up with nothing. What could this possibly be? The Vet thought it might be built up gas. Any thoughts?

I am not sure...did the vet do anything else? If I didn't have any answers, I would seek a second opinion:)

Can the vet refer you to some sort of specialist? :o

One would think if it were gas, it would be a short-term thing. I would be looking for another opinion. Did he check the spleen .. that is located right in that area?