Could be your rotator cuff, could be impingement at the coracromial joint, could be your biceps tendon comin' loose. Look up some rotator cuff/shoulder exercises on the internet, and start doing those (and do them with light weights -- it's therapy, not getting all burly). Get on some anti-inflammatories as well (vitamin-I, Aleve, whatever suits your fancy), ice it after working it, and for goodness sakes, take it easy on the thing, or even take a couple weeks off. If it ain't feeling better after a few weeks, go see a doctor. Chances are he'll prescribe more of the above or some more specific exercises, but he could discover that you're hosed and in need of surgery. Good luck!