Hello. Is your hen coughing and sneezing? Isolate her and make sure to keep your eyes open for any symptoms in the other hens. Do not add more hens for now, I’d assume.
Adding a form of medicine in the water 5-7 days can be beneficial, although hens pose risks of carrying mycoplasma without any symptoms. I’m not an expert, but here is a really informative thread.
https://www.backyardchickens.com/threads/mycoplasma-what-to-do.1621847/
Do you maybe have pictures? There are many experts you can message on this site. Let me know if you need help finding them