Determine which shoulder it is first, right is the likely one for this because the organs are on that side of the body. You could get a scan I suppose buts its a massive faff with the NHS sometimes and you can look up other sypmtoms for each organ yourself instead. You could even try some accupuncture points for those organs and see if it gives some relief then take anti inflammatory herbs to see if it gets better. Either way a good fix is getting sun exposure as much as possible especially first and last thing for free infrared therapy. A led device can help too but the weather is looking good this week and the sun is always better so give that a go.