This is my first flock to raise from chicks. I have always had adult birds. These are 21 week old Barred Plymouth Rocks. Everyone has been very healthy up until this evening, when I went up to close the chicken house door for the night. One hen is craning her neck over her back as if she is going to preen that area, then all of a sudden she will jump up in the air a few times, then end up on her side flopping on the floor. She cackles during this, and it seems to be happening every few seconds to a couple of minutes apart. It only lasts for a few seconds each time. I also observed her walking backward unsteadily after one of these episodes. Between episodes, she either stands very still or slowly walks around. She seems to be very disoriented in between episodes.This is the smallest bird in the flock, but she has been from day one. She doesn't appear to have lost any weight, and this just started today. None of the other chickens, so far, are exhibiting any out of ordinary behavior. From what I've read, it idoesn't quite sound like wry neck to me. We had a deep snow and they have been in the chicken house for 3 days now. I go up and open their door for them every morning and lure them out for a bit with scratch grains in the afternoon, but they all go right back in when they clean that up. Our roo is very amorous and has been pestering the ladies even more, out of boredom. Some sort of strange reaction from (possibly) being over bred? They have been eating Dumor brand organic layer crumble and a bit of scratch in the afternoon late, since it has been so cold. They get fresh water daily, and I have been putting apple cider vinegar in it from day 1, a couple times a week. Their house is the top story of a 2 story root cellar. It is a very old building, so has lots of ventilation. I keep straw on the floor for them and in the nest boxes. Any ideas?? Thanks in advance.