Muscle pain on one side of the body often points to two possibilities. First, you may be doing an activity over and over using one side of the body. This could be something like vacuuming, painting, filing, or even a sports activity like tennis or golf. Try to think back over the last week about any activities that would increase the use of one arm over the other.
Muscle pain can also be referred from somewhere else. Often the neck is the culprit. Pressure on the spinal nerves as they leave the neck can cause muscular tenderness or pain in the neck, upper back, or upper arm. Are you having numbness, tingling, or other changes in sensation on that same side? Have you noticed any weakness of the shoulder, arm, or hand on that side?
A doctor or physical therapist can conduct a screening exam to find the problem. Tests of reflexes and strength may point to a pinched nerve, a condition called cervical radiculopathy. If your symptoms don't go away in one to three days, seek the opinion of a health care specialist.