Usually swimmer's shoulder refers to rotator cuff muscle soreness, inflammation, or tendinitis. Often it is the supraspinatous muscle that is involved. The most common reason that this develops is because of improper stroke mechanics. During, the extension phase of the arm into the water, a greater degree of shoulder roll prevents the rotator cuff muscles from getting squeezed by adjacent bony structures.
More room means less pain.
Shoulder roll refers to dropping, or rolling down, the shoulder of the arm during entry into the water, and during the power phase. Shoulder roll also assists in better arm extension. The shoulder of the opposite arm, during the recovery phase, is pointing towards the sky. This makes recovery easier. The cure , therefore, is stroke drills.