It easily could be allergies. I know our Willow has an allergy to SOMETHING outside. Her reaction isn't scratching, but her eyes swell up and look just awful. I took her to the vet early this spring when I first saw her eyes, and she got drops that cleared it up. She was fine for a week, then it came back. A few more drops and all was well. Shortly thereafter, she went to stay with my parents for 10 days. Her eyes were fine, but the day she came back to our house her eyes swelled again! She has to be allergic to something growing nearby. The heat came when she came home, so the next day we closed up the house and turned on the AC. Within a few hours, her eyes were back to normal!
It's a good thing she's not a dog who enjoys being outside--she stays inside almost all day long (except when I make her go out to potty)--so she's fine with the AC on.
Cedar (tri), Willow (blen), & Holly (ruby)