2 pullets, tails down, legs are shaking

[chickenladytx]

Two of my pullets in the last few days have been showing signs of stress lately. They hold there tail down most of the time, stumble, and when walking and standing they are shaking. They lay down alot. Their eating and drinking is good that I've noticed but they seem to be getting weaker, not standing any longer than they have to. I even caught one today in the big outside pen, she was real unstable and can't run very fast. Not sure if it's that cordiss (something like that), you treat it with corid. Any help or could it be something else? No other chickens are showing any signs like that yet.