The sudden appearance of eyelid swelling may be caused by eye allergy, blepharitis, mosquito bites, eye trauma and so on. 1. Ocular allergy: After contacting allergens or eating allergic foods, allergic reactions are induced in allergic patients, who may experience eyelid swelling, accompanied by eyelid congestion, eyelid itching and other uncomfortable symptoms. 2. Blepharitis: Blepharitis refers to suppurative inflammation of the eyelid glands. Under the stimulation of inflammation, patients may experience redness and swelling of the eyelids, pain, and increased eye secretions. 3. Mosquito bites: When mosquitoes bite the skin around the patient’s eyes, the substances secreted by the bites will cause perversion of the surrounding skin tissues, and the patient may experience discomfort such as swelling and pain in the affected area. 4. Eye trauma: If the patient’s eyelid is bumped, squeezed and other eye trauma, the patient may experience eyelid swelling, eyelid pain and other uncomfortable symptoms. It is recommended that patients with definite eyelid swelling and other discomforts consult a doctor to evaluate their condition and follow the doctor’s instructions for treatment.