There are quite a few here on BYC that are on lockdown fro H5N8 I'll link you to that thread as well.
How are you keeping them? My first thought is since you are having to keep them housed (not sure what your rules are) that there may not be enough ventilation(?). Sometimes if they are "cooped up" and not enough air circulating to keep ammonia and dust levels down, this could irritate the respiratory system.
If you have another place where you can separate her so you can monitor her symptoms more closely that would be good. Offer her some poultry vitamins in her water and give a little extra protein like egg or tuna.
Some symptoms of respiratory illness are coughing, sneezing, wheezing, runny/watery eyes, mucous from the nostrils. If you have any of those symptoms then investigate further. Sometimes with respiratory illness antibiotic treatment may be in order to help control any secondary infections that appear with illness, but you will most likely need to consult a vet.
You can try an OTC "remedy" like VetRx. I have never used it, but some people report that it is like Vick's Vapor Rub (mentholated rub) for chickens. If she doesn't improve when moved to fresh air/being separated, then that would be the first step to take (IMHO) unless you see mucous, runny eyes or respiratory distress.
This thread is active, so I'm sure some folks on there may have some ideas too:
https://www.backyardchickens.com/t/1144502/is-anyone-locking-down-for-avian-flu