Deltoid Soreness

after i pitch my arm is never sore except for one thing, my deltoid. i was just wondering what yout think the problem is. is it my strength, my flxibility, or my mechanics? if it is a problem with my mechanics, my mechanics video is located in this thread:

http://letstalkpitching.com/phpBB2/viewtopic.php?p=54001&highlight=#54001