I would go ahead and treat with the fenbandzole for 5 days, although gapeworm is rare. A respiratory disease such as infectious bronchitis, MG, or ILT might be the problem. MG is treated with Tylan, tetracycline, or Denagard. You can get Tylan 50 injectable at a feed store and give it orally 0.25 ml per pound 3 times a day for 5 days. If she has IB or ILT, those are viruses, and will have to run their course over several weeks. ILT may cause arching of the neck and bloody mucus may be coughed out from the beak. Here is a good link to read about those and other diseases:
http://edis.ifas.ufl.edu/ps044