If she was playing in the field she could have either been poked in the eye by something and that irritated it. Or worse, she could have something stuck in her eye. If it doesn't seem to be getting better by tomorrow you might want to take her to the vet. Can you look in her eye to make sure there isn't something in it? You said it isn't causing her pain, so maybe she just irritated it with something from the field.

I hope she is better soon!