Isn't it unusual to to get very slightly swollen lips from seasonal allergies?

Somewhat: Many allergens can make the lips swell, it is often difficult to determine what it is. It is called angioedema. Front line treatment is usually antihistamines, benedryl for immediate help or zyrtec (cetirizine) for preventative measures. Ice packs can reduce it. You can be treated with Prednisone or ace inhibitors as well (blood pressure meds). Most dentists would be able to help or allergy md better.