As the sage said, you probably injured yourself on ONE hard shot, and the compounding effect of dozens of normal shots afterwards just added to the injury.
I can say I hurt my shoulder the last time I hit. After playing 2 slow crawling sets in 90 degree heat, I went to hit with one of my strong hitting 4.0 buds. Crushing hard groundies both sides for 20 minutes, I hit the ground on one, dropped the racket, and howled in pain. The entire inside of my deltoids seem to have exploded. I still can't lift my left arm up to my shoulders, or brush my teeth, comb hair, or pull off a T-shirt with my left arm.
I'm playing Friday, and it will heal enough so I can serve 70%, usually good enough to play better than how I normally play.
Gotta go windsurfing now, the heat spell broke, and the winds should be in the 20's for sky high jumps and railing jibes.