chicken not doing well- need help

[DeepFried]

I have a chicken that is 19 weeks old. Is acting week (I noticed this since all have roosted religiously since 6 weeks of age but she started to try to sleep in a laying box the last couple of nights- she is too weak to fly up), so I did some inspection. She has a sore/ scab just under and on her vent. She still can poop... but is squeezes out above the sore area and is creamy looking. What does this sound like? What to do.

 

[DeepFried]

?

 

[DeepFried]

Well, the hen was so bad off this morning that I had to put her down (she couldn't even stand up).

Does the original description sound familiar to anyone, in case it is contagious, so I can treat the others.