I just wanted to update this thread in case it helps someone with a similar case.
It was fowl pox. It lasted about another week after I posted. The bump pictured got worse before it got better. Her eye was entirely shut for about 2 days. For treatment I applied iodine and triple antibiotic once a day. I also put a small amount of triple antibiotic in her swollen eye. There was a bump starting on her other eye, but it disappeared when I started applying iodine.
I also put chick electrolyte in a small water feeder in the coop. And hand fed her feed/treats after doctoring her.
Today she is completely healed and is very friendly. I think she's greatfull for the treatment. No other chickens got it so far.