Big, black embedded circle on skin below vent

[zowieyellowflame]

Hello,
I have a hen that has a big black embedded circle below her vent. It kind of looks like poop but it is firmly attached, the size of a quarter, sunken center. Looks like a big scab? There is also some poop accumulating to the feathers on her rear. Tonight, for the first time that I noticed, she was a bit hunched and huddled. When I picked her up she went crazy though. Any thoughts? I put vasoline on it.

 

[ADozenGirlz]

Any chance you can take a picture of it and post it? That's a HUGE help in narrowing down these things.