The last few times I've played tennis my shoulder has had sharp pain when I serve. It's a sharp pain about where the shoulder and arm reach each other. This does not occur on any other of my shots, besides over head/serves. It also doesn't hurt if I serve with out a tennis ball. It hurts when I make contact with the ball. I use a standard continental serves grip and I just hit flat and top spin serves. Now I don't think it's an over use injury because I've only played 5 times in the last two months. I've just started up again because the season is about to start. I'm 18 with a single handed backhand and an eastern forehand. If you need any other info just ask. What could this be? I've never really had this problem before.