It certainly sounds like that swelling could be the problem. It could be a hernia which can get serious very quickly. I would get the vet up to have that checked.
I know you have had her scoped, but you should get her re-scoped (if the swelling on her belly turns out not to be the issue). I would want to know if that piece of wood was dissolving or starting up a chronic infection! If you have an ingrown nail that isn't removed, it is going to continue to in-grow and cause a chronic infection. A low level chronic infection might not show signs until it is too late.
One other question- why can't they remove the foreign body by endoscope? Do they not have the right equipment? Or is it too large to be removed by endoscope?