My left is slowly drifting................?

My left has been drifting under my armpit for the past few months. It is very strange, and i have lost all my . Criss crossing the straps doesn't seem to do the trick anymore, and i don't know what to do. is keeps dissapearing, then it will drift back for a day. then i was up the next morning and its gone again. Im really scared, is there something wrong with me?!?!!!?! im only 21.