Since you mentioned it's been very wet, this could be a coccidiosis outbreak. It's simple and easy to go ahead and treat for it on the off chance this is what's causing her problem.
Get some liquid Corid from the feed store. It will say it's for cows or other livestock, but it's good for chickens, also. Mix two teaspoons of Corid into one gallon of fresh water and give it to all the chickens as their only water source. Mix it fresh each day and do it five days running.
Give it one week and do it again for five more days.
Meanwhile, if she improves, we will be able to conclude that coccidiosis was her problem. If she doesn't improve, then we need to consider something along the lines of an avian virus such as Marek's or avain leucosis causing tumors on her joints or even her brain or spinal column.