A suddenly sick chick, listless behavior though eats from hand, weak, and sits inert and leans to on

Hi there, I have a small mixed flock of bantys, all between 8 mos. to a year old. Yesterday one of my japanese banty's caught my eye because she had not flown up to the roost with her brethren when I was putting them to bed, but was hanging out at the bottom of the coop with the 2 silkies (unusual behavior.) She spent today free ranging with the flock, and came back to the pen close to dusk with everyone, but didn't go into the coop like the rest. She didn't budge when I approached her, and didn't protest being picked up ( both unusual behaviors for this girl.)

I segregated her in a warm place inside my studio, and currently have her in a pet carrier with water and a little food. I did some online research and did the following: checked her vent for parasites and obstructions, neither of which were the case. Rinsed it with warm water just in case i was missing something. Skin look healthy and pink. Checked her all over for injury: none. Felt around for her crop, which didn't seem hard (if indeed I got the crop!) But when i set her down she just lay there, and rolled over on her side a little. I got her to drink a little by putting her beak near the water bowl, and she seemed happy to eat some meal worms, especially wet mash of them. Assuming she's still with us in the morning, what should I do next? I am very new to all this, and lost 2 other chickens over the winter to similar behavior, but at different times and nothing that was passed on to the other girls. Help please!