My baby is 3 weeks old. He is great at latching on, and feeds well so far. At the end of feeds he had been letting go of the nipple, but has started keeping his mouth closed and shaking his head from side to side, or suddenly throwing his head back with the nipple still in. As you can imagine, this is both surprising and painful. Is this normal? any suggestions on why he would do it or what to do? Please?