Do you have photos?
It does sound like you are dealing with a respiratory disease that is spreading through your flock. It would be a very good idea to get some testing to find out which one. Your state lab can do that or you can go through an independent lab. Mycoplasma, Infectious Bronchitis, Infectious Coryza and ILT are a few of the more common ones.
If she is getting a little better with antibiotics, then you likely are dealing with a bacterial disease like Infectious Coryza and/or Mycoplasma. Antibiotics will treat
symptoms of respiratory disease, but there is no cure. Do keep in mind that respiratory diseases make birds carriers for life (those exposed and those that recover). If you bring in new birds, they would be exposed and become carriers as well, even if they never show symptoms.
If you have multiple birds, separate out those that are symptomatic and treat them. If the Sulmet is working, you can try that or if you feel you need something else, Tylosin powder can be ordered online and used in the water. I'm sorry that you are dealing with this.
http://edis.ifas.ufl.edu/ps044