At this age, some babies do shake their heads as they start exploring movement and body awareness, so it could be nothing to worry about. That said, given how frequent it is happening and that it occurs both day and night, I think it would be good to bring her to the PD just to get it properly checked out, for your own peace of mind if nothing else. Better to have a professional take a look than to second-guess yourself, especially since you are the one observing her daily and something feels off enough for you to ask.