What is the reason behind sudden swelling of the eyelid?

Angioedema or a bite: Could be infection, insect bite or sting. Eyelid swelling is also often due to angioedema, probably best explained as a hive of the eyelids. When hives affect the eyelids, lips, hands & feet the swelling is often deeper, more generalized, & can hurt rather than itch. This is called angioedema. The cause is usually an allergy. Common triggers are foods, medications, bee stings, and animal danders.

Swollen eyelid: Another cause could be preseptal cellulitis, whereby an small infection, such as a stye, starts to spread throughout the eyelid. This needs to be treated right away wtih oral antibiotics, since this infection may spread into the orbit and brain. See your local ophthalmologist immediately if you suspect this cause!