Those symptoms could be from a lot of different factors... do you have a spot where you can isolate her? Whenever we have a chicken that looks like it's ailing, or is just acting "different", we put her in the master bedroom shower with a heat lamp, straw, food, and water. Just to put things in perspective (we're not crazy!), we're remodeling our master bath so the shower is unusable right now anyway, chickens aside. I put apple cider vinegar in the water, or if the chicken is really lethargic, one of those electrolyte/vitamin packets. It's not unusual to stop laying at this time of the year (unless you use a light in the coop), but the weight loss and diarrhea are concerning. Try putting her somewhere warm, dry, and safe, with some electrolyte water.