One of my hens, who seems healthy and active, has stopped laying eggs. The weather has been fine, never getting below 30Fat night. She stopped around the time of a cold front a week ago. It was 10-20F for about three days. My other hen has laid the same as before the front, and they all have water and food. She shows no signs of sickness or stress. When should I begin to get worried? She is a year old in March and is a good eight lb. chicken.