What kind of dog is he/she? I had this problem with my boxer a couple of years ago.
Vets had no idea so off we went to the animal hospital. Me being the worried owner agree to an Ultrasound(£500) which showed something in his stomach so they wanted to put an endoscope down his throat which I agreed to. That day I got a call from Doctor to say they wanted to open him up for a better look and as he was still under anesthetic I had to make a quick choice.
They took biopsies of his stomach and intestine they also removed a node from his stomach.
Some big expensive tests were carried out with no unusual results. Then they found some dodgy bacteria in his intestines and presumed that was the problem.
With hindsight I wish we hadn't went down the surgery route. I wish we stuck to finding a medication cure before cutting him.
He ended up on Steroids and Royal Canin sensitivity control food for ages but I weaned him off the steroids due to side effects and the fact they were killing my dog slowly.
How are things now? Well we're still on the Royal Canin food(£60 for 14kg) and no sickness(Apart from if he eats a stick or something)!!!! THIS FOOD HAS TRANSFORMED MY DOG!!! Its expensive but its amazing!
Insurance bill to date £2800. Thank god for dog insurance, Although I really do believe the Animal hospital was rubbing hands when they heard he was insured it was the first question they asked when we walked in!!