IF it is Infectious Coryza she will have it for life, AND will give it to all of your chickens. They get better for a while, then get bad again, and so on for the rest of their lives. They will not ever lay well because they feel like crud. Our state vet told me to kill everything we had, disinfect EVERYTHING with bleach, wait a few months and start all over again. That's what we did and ever since we have had no trouble, because I do not buy birds, only eggs. It is passed from chicken to chicken and it takes a few weeks to show up, so if this is what she has and has been around your other chickens AT ALL they probably have it, but symptoms don't show for a few days to a couple of weeks. This is a miserable exsistance for her no matter how much medicine you give her.