Questions. When you pry her eyelids open how does her eye look? Is the Iris (the colored part) clear or does it look red/green/grey . Her conjunctiva. Is it white or red. The pupil (the black part) is it dilated or pinpoint? Is it perfectly round or irregular shaped.
The reason I ask is because there is a reason she is keeping her eye closed. The eye is painful or sensitive to light.
I have ocular Marek's in my flock. It's latest victim is a pretty and friendly Buff O/Welsummer cross hen who presented with a swollen eye that she kept closed. DH who is a retired eye doctor looked at her eye for me and said it was a herpes infection Herpes can cause a uveitis that can cause the swelling in the eye. It is quite painful. Her eye at the onset of the infection was a deep blue color, almost black that faded into the classic Marek's grey eye.
I sincerely hope it isn't Ocular Marek's but it is a possibility and if it is your whole flock has been exposed.
Here is a picture of one of my hens with ocular Marek's disease. You can see while she has her eye open, the eye it'self appears to be bulging.
View attachment 1310256
This is a textbook example of Ocular Marek's. Grey colored Iris with irregular shaped pupil.
There is no lab work or treatment for the disease if that is what she has. When one of my birds has symptoms, I keep the eye lubricated with an otc eye moisturizing drop like Refresh and if they seem to be in pain, I give them a baby aspirin.
Usually people diagnose the OC just by symptoms and the appearance of the Iris and Pupil.