I'm not an expert, so can only tell you what I would do, absent veterinary help.
I would isolate her (crate, porch, whatever you do) and treat. If you see improvement, probably she'll look better to some degree after 24 hours, then I would go ahead and treat the flock. With corrid, if she is not drinking, I would use a syringe and put the drinking water/corrid mixture in her at first. Just dribble in her beak and she should swallow. Don't squirt it into her lungs. If you don't see any improvement, I would worm. I have had sick birds that I did this with in the past. It may not be optimal, but in the absence of a way to determine EXACTLY what is wrong (ie: a vet) you have to make your best guess. If she has never been wormed, that is also a very likely issue. And again, if you think it's worms, I'd do the whole flock. Either way, they've all been exposed to the same stuff. And with worms, you really need to determine a regular schedule based on your geography and climate for worming. She's at about the age I worm chicks for the first time (6 mos), which is why I went with cocci as the first guess. But honestly it's possible it's either. And they can be carrying worms with no signs at all until they get sick.
I use safeguard goat wormer, and valbazen alternately for worming. I have to worm every 3 to 4 months due to my climate. You may be able to just do a couple of times a year. And you will have to dump eggs for 10-14 days after treatment. There are lots of posts on here about worming, to help you determine what you need to do. Best of luck.
