@Mhamm gave you very sound advice. While it could be a lot of other things going on, the shoulder is a good place to start. Sleeper stretch is good to try; make sure you feel the stretch on the back of your shoulder when doing it. You should never feel front of shoulder pain with this stretch...if you do, you're doing it incorrectly.
Legs (quad strength, glute strength) and core strength are very important. Flexibility is arguably more important. You will need to both maintain strength and flexibility during the seasons of play. Use the off-season to really improve these areas.