Keep her separated from your other chickens for a minimum of 30 days. She sounds like she could have symptoms of a respiratory disease such as infectious bronchitis, MG, or another. These things can be very bad to expose your flock to, since they can become carriers for life. It's unfortunate, but some people don't mind selling birds who are sick. Any stress can cause symptoms to appear. I would get her tested if you can contact your local extension service, or your state vet.