It could be a respiratory disease. Any sneezing, coughing, or nasal discharge? If so, or if these symptoms continue, I'd start her on antibiotics. Oxytetracycline, sold under names such as Terramycin, LA 200, Duramycin, or Tetroxy HCA-280 would be a good antibiotic to begin with. If this is is a respiratory disease caused by a bacteria, such as Chronic Respiratory Disease, the antibiotics should help. On the other hand, if it is a viral disease, like Infectious Bronchitis, the only thing you can do for her is give her electrolytes, probiotics, and vitamins.
Another possibility is Gapeworm. This is more unlikely, though. If it is, treat it with SafeGuard or Valbazen dewormers.
Could she have gotten poisoned by something, such as moldy feed? This might cause labored breathing/respiratory distress. Or, did the older hens jump on her or injure her? They could have damaged her air sacs or internally injured her by doing so.
Keep us updated, and good luck!