Mary,
If this suddenly came up, and this is generalized swelling around the eyes, the dog may have been stung by an insect and is having an allergic reaction. Assuming your dog weighs about 10-15 pounds, give 1/2 of an adult Benadryl (25mg tablet) now and in 8 hours.
If it has been there a while, it could be mites, allergies, bacterial infection.... needs to be seen by a vet. (The Benadryl won't hurt and may help).
Dr. T
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