Haunched and slow moving chicken, help!
Featured Stories on BackYard Chickens
The first thing that comes to mind is egg bound -- that is where I would start on the process of elimination for causes of her distress. You need to get your hands on her and start with palpation from the outside to see if you feel a mass around the vent that would feel like an egg. From there you can wash, glove (if you like) and lube a finger - insert it gently into the vent and see what you feel. You can clean her vent and begin helping to resolve potential egg binding by giving her a nice soak in some warm water (with epsom salt if you have it).
If you do not feel an egg from inside or out then we move down the list and on to the next possible issue -- cleaning her up and getting your hands on her is the first step.