What could be causing my upper eyelid to be swollen?

Infection / allergy: Swelling of the upper eyelid could be from an infected oil gland or skin glan, from a blocked oil gland (stye or chalazion), from allergy, from an insect bite or exposure, and from edema related to lying down. Infections are usually painful, tender and diffuse, while a stye is more like a non-tender lump. Allergy usually is associated with itching. Also, blepharitis is a chronic low grade infection.