First thing you want to do is isolate her from the others like you have done. Inside, in a quiet place, kind of warm and observe her. See what her poos are like and take action from there.
If she has been low on the pecking order and kept away from food and so on, she may not be the strongest immune wise, add that to bad weather and she could have gotten sick. Isolating her will prevent it from spreading IF it is communicable. If it is cocci, which is a parasite found in ALL soil, her immune system could have been so low that it finally took her over. Cocci in small doses is not a problem, since most have a resistance, but once they have their immune system compromised, things can go down hill fast.
See if you can't give her vitamins and electrolytes in her water, and boil up some egg yolk for an extra energy boost if she will eat it.
Chances are, if the others are still up and going strong, they will be fine, but do get her away from the others just in case, since like a weak link in a chain, can cause the load to drop, a weak bird can become the center of illness even if the reason for the illness has always been there an is only being bad becuase she was weakened otherwise.
Best of luck, she's a girl from the looks of it.