What Causes Stiffness On The Shoulder Along With Pain?

I'm 20 and have had no injuries. Off and on for the past two or three years my shoulder has become rather stiff and feels as though I need to pop it. I try to rotate it over my head and back into normal position but it makes a popping sound and just seems to hurt worse. The pain is located in the back area of my shoulder and not on the bone. Sometimes the pain effects things all the way down to my ability to write. The pain has gotten a lot worse in the last few months and I'm not sure what to do for it. Ultimately I want to know is there anything wrong and if so do I need to be concerned and what could it be?