It looks like it could be an abscess. She might have gotten something in the eye or pecked. Keep an eye on it. It might be a good idea to call a vet about it.
I had an iguana that had one like that. It was a small surgery, but it cleared up really fast and well once treated.
It looks like its putting pressure on the eye. I would definitely be concerned for the health of the bird. It might interfere with breathing as well.