Sounds like she's not getting to food/water when she's back with the flock.I let the chick out with the others after about a total of 5 days. She was acting normal, eating, drinking (escaping), etc. She was still slower and not as quick in her mind, though.
Stress can bring about symptoms as well.
I'd still go to Walmart or CVS, etc. and get a bottle of Vitamin E and B-Complex. Dose her as posted earlier.
I'd work on hydration and try vitamin therapy.Give her 400IU Vitamin E and 1/4 tablet B-Complex daily along with some egg.
See that she's eating her normal feed, try it dry or wet, whatever she likes.