mamaChicken 13
She could have a problem with her crop, if she ate too much &/or too quickly. When my hen did the vomit-like thing, it was a 90'F day & she simply drank too much, so some water came back up. A few moments later she was OK. I recommend listing the symptoms in the forum under "Emergencies, Diseases, & Injuries" section - especially if you see other things that seem a little different than her normal behavior.
I've only had chickens for 2 years, so not very experienced. I've seen a sideways, quick head move & yawning as ways for the birds to adjust their crops. if they're doing it a lot, over several hours, or show other symptoms, you should research crop issues. If there's lethargy or an odor, it could be serious if not treated. Chickens (like most animals) try to hide any illness/weakness, so the little things you notice could be a good clue for possible illnesses.