Chicken holding her head and neck in an S shape. Whatever can it be?

I have had my chickens for 4 months, and we have settled into our little routine, ie they totally dominate my life .But today one of my ex batt ladies is holding her head & neck in a sort of S shape. She is running about and is interested in food, but she appears to stagger every couple of steps. I have felt her crop and it is not full. I am now wondering what else it can be, and what to do. I feel so helpless. Would appreciate any thoughts or  advice. Thanks

When you say S shape, do you mean vertically, so she is pulling her head back more toward her body, or horizontally  so that she is holding her head at an angle?

For clarification

If you can get hold of her, gently open her beak and have a sniff.

Chickens can pull off all kinds of contortions when thery are moving food from their crop to the gizzard; but if this has been going on for some time, it may be that she has a slight impaction in her crop.

Have a feel of her crop, and see if anything feels fibrous - because most impactions are caused by grass/straw/hay balls - and then I can give you more advice