Acute pain in the neck and/or shoulder

The phrenic nerves pass through the upper abdominal section of your body, all the way up to the upper vertebrae of the neck. When these nerves get pinched, they cause pain along the phrenic nerve's path and depending on which side of your body hurts, you can diagnose which phrenic nerve is being affected and how to fix it. Additionally, phrenic nerve compression can even cause rotation of the vertebrae in the neck, with the potential to create herniated discs out of nothing.

   ______       _   _____  _____   ______   ________
 .' ___  |     / \ |_   _||_   _|.' ____ \ |_   __  |
/ .'   \_|    / _ \  | |    | |  | (___ \_|  | |_ \_|
| |          / ___ \ | '    ' |   _.____`.   |  _| _
\ `.___.'\ _/ /   \ \_\ \__/ /   | \____) | _| |__/ |
 `.____ .'|____| |____|`.__.'     \______.'|________|


Right [shoulder/neck/upper abdominal]: it is likely that your gallbladder is pinching the right phrenic nerve. The cause for this might be a gallstone clogging up the main duct.
Left [shoulder/neck/upper abdominal]: pancreas inflammation might be the culprit. You could even have pancreatic cancer and thus, the lump of cells would press up against the nerve, but this is less likely. It might also suggest a ruptured splenn, as per Kher's sign, which is caused by the presence of blood or other irritants in the peritoneal cavity when a person is lying down and the legs are elevated.

 ____  ____   ___   ____      ____   _________    ___     ________  _____  ____  ____
|_   ||   _|.'   `.|_  _|    |_  _| |  _   _  | .'   `.  |_   __  ||_   _||_  _||_  _|
  | |__| | /  .-.  \ \ \  /\  / /   |_/ | | \_|/  .-.  \   | |_ \_|  | |    \ \  / /
  |  __  | | |   | |  \ \/  \/ /        | |    | |   | |   |  _|     | |     > `' <
 _| |  | |_\  `-'  /   \  /\  /        _| |_   \  `-'  /  _| |_     _| |_  _/ /'`\ \_
|____||____|`.___.'     \/  \/        |_____|   `.___.'  |_____|   |_____||____||____|


Right side: simply massage your gallbladder with your hand. Persist for a few minutes. To fix this permanently, depending on what's causing it, you might need surgery, but always say no to its removal, as the gallbladder stores bile, and without it you won't be able to properly digest fatty foods, which are essential when it comes to the wellbeing of one's endocrine system.

Left side: Massage the pancreas with your hand. Try to move it around gently and see if you can make the pain stop for a couple of hours or only very intermitently, on the offchance it's being caused by a lump of cancerous cells.

It is also imperative that you read the voodoo flossing sticky here: https://pastebin.com/gP3RJuF8 . Acute pain generally messes up joint mechanics and voodoo flossing is specially good at restoring them.

 ____  _____   ___    _________  ________   ______
|_   \|_   _|.'   `. |  _   _  ||_   __  |.' ____ \
  |   \ | | /  .-.  \|_/ | | \_|  | |_ \_|| (___ \_|
  | |\ \| | | |   | |    | |      |  _| _  _.____`.
 _| |_\   |_\  `-'  /   _| |_    _| |__/ || \____) |
|_____|\____|`.___.'   |_____|  |________| \______.'


-look into how your diet might be related to the formation of gallstones, or inflamme the pancreas.
-if the pain doesn't immediately subside after the massage, you can look elsewhere for a solution.
-if both shoulders hurt, you could suffer from both conditions, or neither of them.
-if the pain comes back after a day or two, try to find the underlying cause.