Medical experts don't know exactly why pain spreads to areas away from an injury, according to About.com. However, some believe that the pain signals jump from the nerve in the area of injury to adjacent nerves. This is called referred pain.
The pain signals then travel to the place from where that group of nerves originates, says About.com. Classic cases of this are pain from a heart attack spreading from the origin of the discomfort to the left arm. The shoulder is a common site for referred pain, according to WebMD. Shoulder pain can be a symptom of gallstones, ovarian cysts, shingles and even gynecological operations.