Welcome to the forums!
I'm sorry your budgie hasn't been feeling well. There are a few reasons that can cause inflammation/infection on the eye and it really would be best to have her properly checked by a vet who has experience in treating birds.
Is your budgie's breathing laboured, do you notice a prominent and rhythmic tail bob? Is the actual eye more watery (discharge) than normal, and does she have wet sneezes? Do you also notice any type of discharge coming from the cere (nostrils)?
Since she doesn't appear to be moulting and if there were no attacks where she could have had her feathers plucked by another bird (it's highly unlikely if there are no other signs besides the missing feathers/swelling on just the one eye), then it's possible she might have a serious respiratory infection and you would need to have her treated with proper medication. The fact that her appetite has decreased is also not a good sign, as she needs nourishment to help fight the possible eye infection or whatever other ailment that she is afflicted with.
For now, it's important to try and build her appetite and you can do so by soaking millet spray onto an electrolyte solution.
Below is a link with detailed info, this will hopefully boost her appetite and make her feel a bit better.
You can also try to give her some soft foods that will hopefully give her good nourishment. Egg food is rich in protein and can help your budgie get her strength back. You can make some by hard boiling an egg and mashing it all up and give it to her. You can even mix some finely chopped up veggies into the egg food.