How old is your hen? Do you have any known respiratory diseases within your flock, and/or have you recently introduced new birds into your flock?
Scratch my second question. I skimmed through other threads you have made, and it appears you do have a respiratory illness in your flock, possibly Mycoplasma gallisepticum (MG) based on the symptoms you reported. If the issue is MG, you can treat the symptoms with either tylosin or tiamulin (Denagard). But I also see you live in the UK. Can you get either of the meds I llsted there?
In your other thread, I saw one of your other birds died. If this hen succumbs, are you able to have an official necropsy done? If so, it would be well worth it to know what she died from. Because even though birds exposed to respiratory illnesses remain carriers for life and will infect future birds you add to your flock, there are various meds that can treat symptoms. Tylosin and tiamulin are only effective against mycoplasma, and of course no antibiotics will help viral infections, so it would be extremely useful to know what lurks within your flock.