I dont know what could be wrong with her.. But some chickens are just a little "off" .. We have an all white EE that kind of hangs off by herself and doesnt seem all there.. We call her Doofy
How are her feathers? Are they good and well kept or are they falling out?? Are the other chickens huddling around her pecking her?? Usually one of the first signs of a sick chicken is feather loss