The Piriformis muscle can be a pain in the butt... literally. But it can also cause pains elsewhere if this muscle is in spasm (as it so often is).
Piriformis can cause issues such as sciatic nerve symptoms as well as providing you with a functional leg length difference or pelvic dysfunction.
And as for that pain in your shoulder? Well that may well be due to the piriformis being in spasm causing other areas to compensate.
Think about it. If you have tried fixing your shoulder by working on your shoulder and it hasn't worked, then maybe, just maybe, the issue isn't your shoulder.