It could even be contact with something like sap or something. I would be inclined to try cold water or even a bit of ice wrapped in a cloth to take out the heat and see if the swelling goes down.  I'm sure you have tried, but see if there is anything lodged - like a grass seed? - under lids etc?  The fact the eyes are clear but puffy does sound like an allergy or temp. irritation.
Biscuit has one Antarax per day for his skin allergy, so you can get antihistamine from the vet.
Hope she improves.