Personally I cannot stand a chicken with a dirty back end. Sometimes that just happens. If I were you, this is what I would do. Regardless of whether she likes it or not. I would take her into the house, wherever, wash that butt and blow dry it dry. Have the chicks in a little box away from her so she can't hear them peeping. If you put the chicks into a teeny tiny little box that can keep them warm and you put a towel or something dark over the top, they will be quiet, they will huddle in a corner of the box and be too scared to move around. I think you need to wash her up good. I have a blowdryer that lives in my chicken barn, just for those specific occasions. You could also bring warm water into a container and stand her in the water and use a plastic cup, I have measuring cups I use for this if I can't get into the shop to use the sink there. I keep putting warm water in the cup and drizzle it over the poopy bum. Sit that chicken on my lap with a towel and then blow dry to kingdom come. I have no issue with keeping backends of birds clean and methods to do it. I find if one begins a poopy bum, that the feces builds up and builds up and you got snowballs, well actually poopballs back there then. She probably just had a huge broody poo that was just icky. Chickens can hold their feces for a good long time, something to do with two poop holding areas in the birds back end. Read an article about that, can't remember where it was or the whole gist of it, but interesting. I don't think she is sick, but the probiotics are certainly a good thing for her and hers. Good luck. Keep us posted on what you do, and have a wonderful, chicken poopy bum day!! CynthiaM.