It's possible that the chick is exhibiting a behavior known as "head shaking" or "head bobbing," which can sometimes occur in response to discomfort or irritation in the crop or upper digestive tract. Chickens may perform this motion to try to dislodge something stuck in their throat or crop.
However, since the chick just came home today and immediately began eating voraciously, it's also possible that the head motion is simply a result of excitement or eagerness to eat. Sometimes, chicks can exhibit unusual behaviors when they're adjusting to a new environment or experiencing new stimuli.
Given that the chick seems otherwise healthy with normal droppings, it's a good sign. Continue to monitor her closely for any changes in behavior or signs of distress. If the head shaking persists or if you notice any other concerning symptoms, it may be worth contacting a veterinarian who specializes in poultry for further evaluation.