Shoulder tweak....

I am 16 right now and I have been playing since I was seven. I am ranked and I try and play for about 2-3 hours everyday. I recently switched rackets from a Wilson Triad 6 to the Head i.prestige mid and its fine on my groundstrokes and volleys but yesterday on my serve I felt a twinge behind my shoulder blade. The pain wont go away and when a move around my arm I feel a small clicking(almost). Ive had shoulder problems before, but not in the back of the shoulder(more towards the front in the rotator cuff).

Has anyone else had this problem? Or know of any solutions?

Thanks.

Yeah, I have had this before i think, I used to play that same amount, and one day, when I moved my arm around it started to pop, the pain that I most experienced it was when I hit serves and overheads, never the less my trainer said it had to do with instability in my shoulder, my front muscles in my shoulder were stronger than my back muscles, when I did something strenuous with my shoulder, my shoulder would move out of place a little bit causing pain, I would go see a trainer because the problem will not go away without physical therapy, oh and my doctor said it was because of overuse, not because of a heavy racquet