The loose stool may or may not be related. What color is the poop? Urates? Do the butt feathers smell really musty sour? (Intestinal yeast infections make the butt feathers smell very strong)
Bacterial infections can cause fever. Feel the feet. It they are ripping hot and or the bird is pulling...
If she is contagious, shes already spread it to the others having carried this virus for quite sometime. If she were mine, Id put her back with the others, IF they won't bother her. Lots of vitamins, probiotics, even try some spray on Colidal Silver in hopes of slowing the growth of this thing...
Then its either as Eggcessive suggested some sort of tumor or an ingrown feather. A few types of viruses do cause tumors in chickens, Mareks and Avian Luekosis comes to mind.
As suggested, soaking in Epsom salt might suck the poisons out, a warm compress might be the easier route than soaking...
Could be an ingrown feather. I see a black spot in the middle. Is this a hole with black material in it? If so, this probably is a ingrown feather. And if it is, you would start with an alcohol wipe on the area, sterile instruments and at that hole you first dig out the feather. You will need...