What Causes Shoulder Pain?

Which ligament in the shoulder is causing me some annoying moderate pain? - at the downward curve near the top of the left shoulder toward the back (away from the neck) where the end of the bone feels round (almost hemispherical). The small area is sensitive to the touch. It feels like someone hit the shoulder with a 2x4 yet don't remember hitting anything. I don't think it is associated with any bones (arthritis). I'm 87 and in reasonably good health. What's the best way to treat it?
Thanks
~Ric