system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can a tiny pimple can hurt a whole shoulder?
gpt
This is called "referred pain".  It occurs due to our nervous system being so complex and interconnected that a painful stimulus can cause other parts of the body to hurt too.  This is why heart attacks can cause pain in multiple places like the chest, shoulders, arm, and jaw.

Edit: depending on the extent of the pain, it could just be due to inflammation.  Blood vessels dilate in areas where certain issues are happening (e.g. infection, trauma, etc.) And this influx of blood causes leakage of fluid outside your blood vessels.  This creates pressure in the area, which stimulates nerves and is painful.