What does it mean if you lay on your arm on your shoulder started hurting?

Evaluate shoulder: Shoulder pain can be referred from the neck or it could be locally from the shoulder. Common local problems include but not limited to spasms, arthritis in the ball and socket or ac joint, bursitis, impingement, rotator cuff tear, labral tears, biceps problems, stiffness, and instability. See an orthopedist to diagnose the problem.