Was playing tennis right handed taken a swing at the ball,sharp pain right behind shoulder. what?

Posterior shoulder : 56 yo r handed? Gentleman with posterior shoulder pain. You need an in person full history and physical exam. Will need to determine whether you had an intrinsic shoulder injury such as rotator cuff tear or extrinsic pain referred to your shoulder from your cervical spine or abdomen. See your MD shoulder/sports orthopaedic surgeon if possible please. Until then Rest, NSAIDs, physical therapy asap