Massive black scab under tail, WHAT HAPPENED?!

[kitchwitch]

In the continuing adventures of Lola the NHR I brought her in for another bath due to her poopy-butt.
A couple days ago she had a bath because of her prolapsed cloaca and I soaked her, removed debris and feces and she was right as rain and today she received anther bath because she just looked gross. This time when I was bathing her I noticed a big black spot I never noticed before, it's about the size of a half dollar, somewhat sunken and it's hard and crusty like a scab. I've never noticed any injury on her before and I feel like crap because whatever injury may have had (or may still have) lead to this massive black thing under her tail and I didn't ever notice it. She's never had any kind of movement issues or limp, her comb is a bright beautiful red and she doesn't have eating issues except that she's a bit thinner than the other chickens I have. Any ideas what this could be?
A couple weeks ago SOMETHING got into their run and got my other NHR because all I found of her was a massive pile of feathers outside a small ground level hole in the fencing. Could this something have inflicted an injury on her? This is frustrating and I feel terrible

 

[Indiana hens]

Did u check for lice?

 

[ranchhand]

If she is in with other hens they may be pecking her on the roost at night. They are attracted to blood and also sense when a flock member is weak. Chickens are cannibalistic. I would suggest separating her until she is back in tip-top condition. It might be an old wound from the attack, but if you bathed her I would think you'd have seen it. Separate her, treat with all you can and then reintroduce her slowly.

Do check her all over for any other conditions, very carefully. Good luck, sounds like she is having a rough time.

 

[kitchwitch]

I checked for lice and in the past couple days she's been bathed, powdered, de-wormed and just about anything else you could do apart from getting her tarted up and taking her out for a night on the town. She's separated at the moment, but she isn't bleeding and wasn't when I got her out of the run for her bath. When I had her out last time for her cloaca there was blood on her tail feathers and I was panicked but once I bathed her I realized that there was no new blood and I assumed that the others had been picking at her prolapse. I fixed her up and had her out for a day before putting her back in with her friends where she immediately resumed her role as not-so-benevolent dictator.

While she was out the last time I also cleaned out the coop and hosed it down with hot soapy water threw down some DE once it dried.

I'm just so upset with myself and I feel so terrible for what my neglect has caused her. I'm going to keep her out and keep an eye on her.