Is she a well-nourished chick, or one that is puny and slower growing? A bad heart can affect breathing. Make sure there are no wet shavings or moldy areas since aspergillosis from mold can cause severe breathing problems. Can you describe the breathing in more detail? Any gasping, rattles, wheezing, sneezing, nasal or eye discharge? They can choke on a larger piece of feed or something they have swallowed that lodges in the airway. Did these chicks come from a farm or a feed store? If they came from a breeder, I would call them. If you need an antibiotic for a respiratory disease, LS 50 (lincomycin/spectinomycin) is good for chicks. If you can't find that oxytetracycline or Duramycin10 are found easily.