Sick pullet? Or just odd behavior? Help please!

I have 12 BO pullets who are 12 weeks old. One has been acting strange at least 2 weeks that has been very noticeable. She doesn't strut around or peck or even coo or cluck like the others. I haven't seen her eat. I have seen her drink when I put her near the water. She hangs her head, likes to stand under low areas and doesn't move much. She goes in the house at night and comes out in the morning but does not act normal. She comes over and stands on my feet and likes me to pick her up and she would stay on my lap forever being pet. No on else seems sick. I really do not know if she has always been like this. Being new to chickens, I noticed it as I came to know them better and as they became more socialized. Help. I feel so helpless to help her.

Don't know about her poop. But yes, it's kind of like "nobody's home". Except that she does have an awareness as she goes in and out of the house, seeks to go under things, finds me and stands on my feet until I pick her up. I will try to check poop if possible. I will also check out Newcastle's disease. I also wonder about genetic anomalies like in people, just very odd. Is that possible or do those just usually die. I can't imagine her surviving like this. The others do not pick on her, just ignore her.
 

ps. Her feathers have always seemed strange compared to the others, from the first time they were all fully feathered.
 

No interest in treats. The big treat lately is koscia, the prolific weed in my garden. The others go running to eat it. She has no interest whatsoever.