Of course we can't tell you what's making this little girl sick. That would require lab tests. It could be reproductive, protozoan, bacterial, fungal, or oncological. But I can tell you she is dying. At this point, not active and not eating or drinking, it may be too late.
You could try syringing a drench dose of Corid into her in case this is coccidiosis. There's a small chance it can turn her around if it is. The dose is about .5ml undiluted liquid Corid once a day for three days. I would also give her sugar water to keep her glucose elevated. Stop the vitamins during the Corid treatment as B-1 feeds the coccidia parasites.
Be prepared for her to die, though, probably within the next day or two.
Watch carefully for any of your other chickens to behave lethargically and have runny poop. In fact, getting ahead of the curve by putting Corid in their water may be wise just in case.
If you have a vet, you might ask them if they will run a fecal float test on a random flock poop sample.
