The sudden appearance of a swollen bag on the patient’s upper eyelid may be caused by eye allergy, blepharitis, blepharocysts and other reasons. 1. Allergy: A sudden swelling of the upper eyelid may be caused by allergy. If the allergic patient’s eyes come into contact with allergens such as dust mites and pollen, resulting in eye allergy, the patient may experience eyelid edema, which may be accompanied by itchy eyes and other uncomfortable symptoms. 2. Blepharitis: Blepharitis refers to acute purulent inflammation of the eyelid glands, and patients may experience symptoms such as eyelid congestion and swelling, accompanied by burning sensation and pressure pain. 3. Blepharoceles: Swelling of the upper eyelid may be caused by a blepharoceles cyst. Eyelid cysts are chronic inflammatory granulomas formed when secretions from the eyelid glands are blocked from being discharged and retained. It is recommended that patients with significant swelling of the eyelids and other discomforts seek prompt medical attention to evaluate their condition and follow the doctor’s instructions for treatment.