Dirty chicken butt....help?

I have a chicken, named Matilda, and I don't really know what type she is. (She looked like a chipmunk as a chick, and is slightly larger than a bantam.) Anyway, yesterday I noticed that her butt seemed to be slightly bald around, you know, her bumhole, and the feathers look like dreadlocks covered in poop. I am planning on cleaning her tonight because she is not socialized and that is the only time I can catch her. What should I clean her with? After reading other posts, I am anticipating finding lice or mite eggs, so I know I need to purchase a product to kill them. She is eating and laying eggs normally....do you have any tips/suggestions on what is happening/what I should do?