I have a 4-year-old white Pekin hen who has recently developed a stiff walk. She has always had a bit of a hobble, but it has grown worse over the past few days. She lays down whenever possible and it is noticeably difficult and painful for her to walk. She stopped laying eggs about two weeks ago and has since begun molting heavily (more so than is usual for her). I have checked for Bumblefoot and have ruled that out. She is fed a 50-50 mix of cracked corn and a generic laying mix, as well as foraging. Today, I started putting Niacin and a multi-vitamin in her water (100mg/gal). She is kept with one drake (same age), who shows none of these symptoms. Any ideas?