I hear you about the cost thing!
But as Ryle said, go ahead and have the vet out. I just had my saddlebred worked up on...intermentant lameness that no one could figure out.
He gave him the full lameness workup - flexxion tests, trotting in a straight line, trotting in a circle - etc. Also, blocked nerves and took x-rays. All of that was only $140 - and now, my roany pony is back!
All of that to say "Go ahead and call the vet" - you'll probably get a very clear idea of what's going on with a plan to get him on the road to recovery.