check the hinge screws, they will work loose over timee, also, it may be settlement in your foundation, if the screws are tight, recommend calling in a pro to check it out.
some clues to look for for settlement.. are there cracks in sheeetrock walls above the corners of dooors or windows?
if you place a marble, or ball in thee center of the room does it roll on its own?
when the door is shut,, is the margin, or area around the edges of the door even?