Welcome to BYC. Does she seem lethargic or sleepy, having any nasal or eye drainage, or have noisy breathing or gurgles? Do you see any peck marks where another chicken has pecked her face? Respiratory diseases such as MG or coryza can cause swelling around the eye and face, but pecking wounds can cause the same swelling around the dace. Research "common chicken diseases" for a good link to read from U of Florida.
There looks to be a peck mark or a sting mark about 9 o'clock in your picture. I would bet that she was pecked or stung by something if she isn't acting sick. You might try giving her 1/4 of a 25 mg Benadryl tablet to see if it helps the swelling go down. If you start to notice pus or drainage out of the eye, then I would probably start an antibiotic, or use some antibiotic ointment in the eye after cleaning it out with saline.