You're not being paranoid - it sounds like something is up. I can only suggest what I've done for mine - I'm not that experienced, and I just research a lot when I have a problem and then give it my best shot. There are many folks here with loads more experience and knowledge than I have.

Are they on medicated chick feed?
She may have coccidia and whatever she has, they will probably all have it soon, if they don't already.
I would give her some vitamins/electrolytes in the water ASAP and make sure she's drinking.
TSC sells something called Chick Saver you can try. I'd also try to get her to eat some plain yogurt, for the good bacteria for the gut, or use Benebac
The medicated chick starter is supposed to keep coccidia in check. If they are on plain chick food, I would probably do antibiotics in the water, rather than just putting them on the medicated food; the levels in the food probably aren't enough once they show symptoms.
1/2 tsp Corid powder mixed with 1 gal. water [I've read lots of different opinions on dosage so that's just the one I chose to try]