What exercises can I do to help a sore shoulder?

For about 6 months I have had a sore shoulder. My doctor says it is not a torn rotator cuff and is probably muscle soreness or bursitis. It doesn't seem to be swollen, just very tight and it hurts when I reach overhead. I take martial arts and we do a lot of push ups and punching. After doing some research I believe my problem might be that my chest and shoulder are too tight, pulling the shoulder forward and not allowing it to rotate properly. Does this sound likely? If so what are some "pulling" exercises I can do to strengthen my back without a weight machine?