It sounds like this chicken is wasting away. There could be a number or reasons for that.
Not eating is the obvious one either because she is unwell or being bullied.
A disease or virus.
A blockage in her digestive system.
Worms or coccidiosis
are other possibilities
More information about her would be useful...
How old is she? Has she started laying yet?
Has she been wormed and was she vaccinated for Mareks as a chick?
What does her poop look like?
Is she eating?
Is she wheezing or screaming? ...The head shaking can sometimes be associated with a respiratory infection. Do her eyes look right? Is there froth in the corners of them?
When chickens get to this stage of hunching up and acting sick they are usually pretty unwell, as they will try hide their illness for as long as possible to prevent being picked on by other members of the flock. Sometimes they will even pretend to be eating but if you watch closely they just pick up and then drop the food or peck at it but not take it into their mouths.
Putting her in a warm place (ideally on a heat pad) should perk her up a little and then see if you can get some nourishment into her.... I find scrambled egg and natural yoghurt with some of their regular feed mixed in to a porridge sort of consistency, is a good way to tempt them. If she won't eat then you may need to consider tube feeding her as she will just get weaker and eventually die if you can't get food into her. Also important to get her to drink of course.
Edited by rebrascora - 10/10/15 at 6:23am