Often, as I am working with clients, the question comes up: Why does my (back, arm, leg, head, etc.) hurt when I do something with another part of my body? This can be confusing and frustrating.
I am currently re-reading a wonderful book called Job's Body by Deanne Juhan. As I was reading, I came across this great passage, which i think helps to explain the "random" pains and discomforts that occur.
For instance, let us imagine ourselves observing a person who is standing erect and executing the simple gesture of raising their straight right arm to the side until it is horizontal. The fibers in the deltoid, the supraspinatus, and the upper trapezius will contract to produce the primary motion, while the fibers of the pectoral major, the pectoral minor, and the latissimus dorsi must simultaneously extend to allow it But the contraction of the right trapezius will not only raise the right arm, it will also tend to pull the neck towards the right; therefore the left trapezius, along with other muscles of the neck, will have to contract as well in order to stabilize it. Furthermore, the extended right arm will overbalance the torso to the right, so the erector spinae muscles on the left side of the spine must contract to brace the whole torso and keep it erect. And since this contraction of the left erector spinae set will tend to pull the left side of the pelvis up as well, the gluteus medius and minimus of the left side must also brace to hold the pelvis level. Since not only the torso, but the body as a whole is threatened with tipping by the overbalancing weight of the extended arm, the right leg must brace as well, using fibers in the hip, the thigh, the calf, the feet, the toes. And of course our subject continues to breathe, so all of the muscles which cooperate to fill and empty the lungs must now make the necessary asymmetrical adjustments to continue their rhythm without disturbing the pose. And to further complicate the picture, if we add a weight (say a book) to the outstretched right hand, even more fibers from even wider areas will have to be called into play and instantly coordinated in order to preserve the position.
All of these muscular event must concur for such a small, isolated gesture, and even this description has been simplified considerably. (page 114)
It isn't any wonder, then, that an injury in one area of the body can cause what seems like random pain in other areas. This is one of the many reasons bodywork is most effective when applied to the whole body, rather than a specific area of discomfort.