My son had issues with his shoulder at the beginning of last spring with his college team. Have you been doing a lot of winter lifting? The baseball team had worked with a football/hockey trainer over the previous winter and he had them doing some lifting that ended up being "not pitcher friendly". His shoulder was just "tight" from the type of lifting he had done all winter. They gave him some specific stretches to do before throwing...and told him to stop doing a couple of the lifts....and the issue went away fairly quickly.