We had a dog that had very similar symptoms that came on suddenly. His exploratory surgery also came up with nothing, and he was eventually diagnosed with inflammatory bowel disease. If that's what it is, blood work would not show much of anything, but it would make me nervous that they didn't do a blood panel to rule out other things.
Will you be seeing your regular vet from here on out? I'm not sure how much diagnostics have changed since our experience (that was back in the 90s) but it took a visit to a veterinary teaching hospital and an intestinal biopsy to finally get the diagnosis of IBD. I know how very stressful it is not knowing what's wrong. I hope you get some answers soon.
