This is very normal for abdominal laparoscopic surgery of all sorts and can last the rest of your life.
This happens because there is a particular nerve that goes up your body from your diaphragm to your spine and the fibers of that nerve (which touches almost every organ) also pass through your shoulders ... and your stomach. When you eat, you apply outward pressure on the pouch which causes the band to apply inward pressure on certain nerve fibers, those same ones talked about earlier, and all of that pushes down a bit on the diaphragm, and all of that combined sends pain signals (referred pain) to your shoulders and back.
Even for non lap band, nerves are damaged in the process of surgery (it isn't done delicately - watch a few videos, it's violent) and when someone eats or moves or breathes deeply, or bends funny or all sorts of things it can trigger that referred and displaced pain.
Unfortunately there's nothing you can do about it. For lap band people, just try not to over eat to reduce the pressure.