It definitely sounds like one of three conditions, and it's possible to be a combination of them and then some:
1) General tendinitis of the tendons around the shoulder.
2) Shoulder impingement syndrome.
3) Rotator cuff issues.
MOST posters here are not qualified to do a diagnosis with just a written statement, and even if they are, that's very little information (about you) to make a proper diagnosis. I think it's best that you go see a shoulder specialist and have it checked out.
Sadly, because of the outrageous ROM and flexibility of the shoulder joint, comes instability. You can have the most perfect service motion and still get shoulder injuries.