Why does my 6 month old baby drop her head down or to the side suddenly then picks it back up? it looks like when a person falls asleep sitting up

If you're not sure: If this is an involuntary movement, video an episode on your smart phone. Include your child's behavior for a few minutes after the episode.Note what she's doing prior to an event, how often it happens, & whether the movement interferes with eating, playing, etc., all useful history for her doctor, along with physical & neurological exams & developmental screening ( see firstsigns.Org).