Swelling under one eye can be caused by a variety of factors, ranging from mild conditions to more severe ones. Here are some of the most common causes:

• Excessive eye rubbing: Rubbing the eyes due to fatigue, itchiness, or a foreign object in the eye can lead to swelling.
• Injury: Trauma to the eye or the surrounding area can cause swelling.
• Blocked tear duct: A blocked tear duct can cause fluid to accumulate and lead to swelling.
• Stye: A stye is a red, painful lump that develops on the eyelid and can cause swelling.
• Periorbital cellulitis: This is a bacterial infection of the eyelid that can cause redness, pain, and swelling.
• Allergies: Allergies can cause swelling under the eyes, along with other symptoms such as itching, tearing, and discharge.
• Insect bites: Mosquito bites or other insect bites on the upper face can cause the eyelid to swell.
• Aging and genetics: Tissues around the eyes can weaken with age and genetics, leading to fat moving into the lower eyelids and making them look swollen.
• Organ failure: In rare cases, swelling under one eye can be a sign of organ failure.

If the swelling is mild and temporary, it may clear up on its own with at-home treatments such as cool or warm compresses, OTC antihistamines, or keeping the area clean. However, if there are signs of infection or a more serious injury, such as pain, inflammation, or pus, it is important to see a doctor promptly. Seek immediate medical attention if there are symptoms of an eye infection or other health condition, such as redness, pain, blurry vision, or loss of vision.