Actually, she is a pullet. A pullet is a female chicken under the age of one (when they become a hen). I'm not sure what it could be. It's good that you have separated her. Make sure you change the bedding and disinfect the brooder that she came from to keep the other chicks protected. How...
I'm so sorry. You will need to isolate her immediately. It sounds like it may be Mareks Disease. Read this article and see if you feel she fits the description?
http://www.helium.com/items/1377323-mareks-disease