Look at this possibility. When eg you move or resize anything and create a keyframe, then go back to that frame later to change the setting, if you dont get the exact frame it creates this problem. You have had an encounter with the dreaded POSE MONSTER. Its got huge teeth and so many mortals alone have encountered this fearsome beastie. Click on something, go to the Window menu and pick Graphs. With a popup menu on the side pick the movement, ie bend. Normally the line is straight (he writes, hoping to include the picture), when something moves in the middle of the animation it creates the spine (middle graph). In the third graph a frame next to the keyframe was picked causing the bulge at the curser (lower graph) in the middle of this curve is where the part is running amuck, ie size. Click the bulge and LINEAR (zig-zag) to get a straight line.