Why is my dog nudging at his side?

He's been pretty sick starting Thursday, he vomited off and on all day long, and stopped eating. Friday he wouldn't do much of anything and actually vomited a piece of plastic with burned edges (he likes to dig around in our trash burning area), then Saturday was okay. Sunday he was back to it again with a little blood in his stool, so we took him to a vet where he started vomiting some blood. The vet ran an endoscope into his stomach which turned up nothing but inflamed, red stomach lining (likely the cause of the bloody vomit), so he went ahead and performed X-rays, which also showed nothing. He stayed overnight, and ate, drank, and acted fine all day today (per the Vet), so he came home. Now he is acting somewhat normal, but he is groaning a little more than usual, and is nudging at his side. I may just be overreacting, but I'm concerned as he has a history of dietary indiscretion and has had foreign body obstructions in the past. Have you ever had a dog with bloody vomit/stool, and if so, what could be the cause? He has had pancreatitis in the past, so I know this is not it - witholding food for several hours didn't help.