I was having this same sensation/pain the first week or 2 after surgery and I noticed, when I took my leak test that when I sipped the barium stuff, the top of my stomach (right where it meets the esophagus) was moving when I got those pains. I don't know if it was a spasm, or just the normal movement of the stomach. My Dr. nor the radiology guy said anything about it, other than everything was perfectly normal. Also, the more it moved, the more air came up from my stomach and I could literally see a burp moving through the barium. I learned that it was when something entered my sleeve that it gave me that little twinge and also the liquid filling my sleeve that made the burp come up. I'm 3 weeks out now and I can't even feel it anymore. I can't pinpoint when it stopped exactly, but It was somewhere around 10 days to 2 weeks out so this should start to get better pretty soon.
Good Luck!