Could she have been stung or pecked? Are you in a warm area with mosquitoes out? Fowl pox sometimes look like that on young chickens. Do any others have any bumps or scabs on combs, wattles for faces? Here is what fowl pox can look like on a young chick: https://upload.wikimedia.org/wikipedia/commons/8/80/Plokes_poyon_do_dzeu.jpg
So, she's a bantam and probably around a year old. I just noticed it yesterday but I had one that had a bit of a swollen eye last week but has since cleared. We don't have mosquitos yet here.. or wasps or bees yet.
I would keep an eye on it. Her eye looks fine, and she doesn’t appear to have any bubbles or nasal drainage. Watch to see if she is being pecked by others.