I was playing for about an hour, and right after a I made a backhand slice, my right (upper) shoulder got stabbed with a sharp pain. Being an idiot, I decided to keep playing despite the pain. Everytime I hit a forehand, my shoulder would hurt like hell. After about 10 minutes the pain went away and I continued to play as if nothing happened. So...what exactly happened? I've been playing for about 5 years and the only tennis related injuries I had were twisted ankles because of careless running over balls.