Bizarre behavior in sick chicken?

I obtained 3 adult chickens from another flock about 6 weeks ago (maybe this was my first mistake). This is my first time caring for chickens! I was told they were between 1-2 years. They all seemed healthy at that time. Over the past few weeks I have noticed one chicken who has become increasingly lethargic. She eats some but not well. I noticed blood in what I think was her poop. She has seemed to have diarrhea much of these 6 weeks. Her feathers around her vent have poop on them.I thought she may have coccidoisis so I am treating with corid. I can't tell much improvement with the corid except I don't notice any more blood and not as much diarrhea. Her comb seems limp and a little pale. Honestly she looks like she is going to die. She also seems to be doing the bizarre behavior where she will stick her head in-between things (like fence posts) and just sit there for a long time. The other chickens seem to be doing fine and are laying. The only thing I can think of that may have affected her is that I noticed some of my rhubarb leaves were eaten and I know these can be poisonous.
Any ideas?? Thank you!