I would seek advice. There seem to be several theories as to the cause of this shoulder pain. Initially, I have read, it can be the C02 gas. Later on, it can be due to the band pressing on the 'phrenic nerve' which is more noticeable when you are full. It can even be a symptom of nerve damage to the diaphragm caused by the band, itself.
There are probably many other possibilities but, as with any other persistent pain that worries you, seek professional help. You need to set your mind at rest as well as, hopefully, to get rid of the pain!
All the best x