Just went down to the barn to collect eggs and give the girls some treats. Whenever they hear me they all (45) come running into the barn from the pasture to see what I'm giving out. While I was visiting I noticed one of my Barred Rocks outside to the side of the chicken door. I stuck my hand out and she did not move. Then I went outside and she was standing still but every few seconds she gave her head a little shake, (not really a tremor but she ruffled the feathers around her neck) I don't know how ese to describe it. I threw her a few treats but she did not even look at them. Please help. If it's something that can run through the whole flock I need to know what to do. Never had a sick chicken. Earlier today all were fine.