hannah is 6 months old and is crawling, she has good head and neck control but latley when on tummy and up on arms she has been letting her head drop forward and then quickly snaps it back up. it looks as if she can't hold it up, like it is really heavy and then she quickly whips it back up. i don't know if this is just a new thing she has learnt and thinks is fun or if she is really having neck troubles. she doesn't seem to be in any discomfort. any ideas?