“I can’t lie on my shoulder at night!”
From http://www.painspot.com/ – (the pain locator tool): Shoulder pain on the side of your upper arm may be due to problems in a space beneath the end of your collar bone in the area above your main shoulder joint. Pain coming from this location often is from bursitis or problems with one of your rotator cuff muscles. It may be especially painful lying on your shoulder at night, or raising your arm when you reach forward to get something off a shelf, or getting a gallon of milk out of the refrigerator.
The rotator cuff muscles of your shoulder are four fairly flat muscles surrounding the main shoulder joint. These stabilize your shoulder and help you move the shoulder in several directions (for example raising and rotating your upper arm.) A small cushion-like sac called a bursa sits just above the top rotator cuff muscle (supraspinatus) and under the collarbone. You may feel pain from this area often on the side of the shoulder, which may be from tendonitis, bursitis or a torn rotator cuff muscle. To see which one it might be, go to http://www.painspot.com/pain_locator/female, pick your painspot, and then take the shoulder pain quiz.