My lo did this, he would lean right down to touch his shoulder and so i took him to the doctors. She asked whether he was sitting ok without falling over, checked his ears and eyes i think and just said to record it and bring him back if it carried on. He stopped doing it about a week or two after and i haven't seen him do it since. It's hard not to keep thinking about something and so it might be worth just nipping him to the doctors for your peace of mind x