I'm no expert on this, but a few thoughts...
Sometimes they move there heads and open their mouths to do a crop adjustment. could be as simple as that.
Did you notice any other symptoms?
Two other thoughts,
1. is her crop full in the morning, is it not draining? is she moving her neck like a snake? if so, could be sour crop or impacted crop.
2. hens open their mouths (gaping) when they have gapeworm, can you open her mouth and take a peek in her throat? I think they are red. But again, I don't have much experience.
Perhaps someone will have some better answers. I know how it can be when you think a girl is sick!
Hope your little girl is fine and this is just a little crop adjustment. Good luck!