Shoulder - possible torn muscle

Hi,
I was using some bolt cutters to cut up some steel mesh when I felt a muscle possibly tear in my inner shoulder right up the top of the torso where the arm connects. Whilst I'm not in constant pain the arm is weaker than previously and unable to do such activities as say throwing a tennis ball as the shoulder does not have the strength and also causes discomfort. It has been approx 12 months since the injury occurred. Whilst there has been say a 50% improvement the issue still exists. Just wondering if it possible to determine what it is I may havedone and if I should be doing anything in particular to treat it ie. is it likely to heal, should I be doing a particular exercise etc ? Thanks in advance for any advice you may be able to give. Jason.