I'm an er doc trying to diagnose itchy swollen unilateral ypper lid in healthy 23 year old of 2 days duration. no conjunctival or lower lid involveme?

Consider insect bite: Where I live, Seattle, there are spiders all over the place. They like to drop down on their silken lines onto the faces of sleeping people and occasionally will take a nip. A lid with little erythema, itchiness and swelling is the usual result of this. Sometimes you can observe the bite marks - two punctures close together, at the center of the swollen lid. Treat with antihistamines.