How old is she? Does it look like its pus filled? Hard or squishy? I know when I washed my girls at the beginning of summer, I don't remember seeing anything like that, but maybe I wasn't paying attention. The one thing I know that happens when its this hot and they have poopy butts is fly strike. I don't know if that's it, but if you are on FB there is a page called the Poultry Pages and it is very active and there are a lot of experienced chicken keepers there. Post the pick and you might get some idea of what it is and how to treat if necessary. Hope she is ok, let us know.