i would guess its the way you're gripping the bow. the grip of the bow should be right on the edge of your hand, so that only the thumb is on the right side of the riser (right handed). having the bow in the middle of your hand often causes your arm to stick out in the path of the string.... its hard to explain without actually showing you. just try to make the grip of the bow contact with the very right edge of your left hand...vice versa for leftys.
Probably your grip on the bow. But if you didn't have your draw length checked before you bought the bow it wouldn't be a bad idea to have it done. Moduals are not to expensive and can be changed pretty quick. Love my Cr337