Shoulders are the weak link in the paddling equation. I have simple exercises and information on dislocations here and then also and for impingement here. but for a more elaborate medical resource, check out the dislocation and rotator cuff pages from this Orthopaedic Surgery and Sports Medicine at the University of Washington informational site.
Are you just experiencing sharp pain, caused by wear and tear rather than one incident? Impingement syndrome is a sharp stabbing pain on the front of the shoulder, about an inch down. This is often caused by poor shoulder positioning (slouching), poor flexibility, and poor rotator cuff control. To avoid or relieve impingement syndrome, a common complaint of paddlers, it is important to stretch the shoulder, utilize proper posture and strengthen the rotator cuff muscles. On the bright side, impingement pain tends to respond quickly to therapy. Go see a shoulder savvy Physical Therapist for a few sessions and a quick recovery! (for me it was just 3 visits and I was as good as new)