Eye swelling is a common medical condition that can be caused by a variety of factors. Two of the most common causes are styes and STY. A STYE is an infection of the eyelid caused by bacteria, usually Staphylococcus aureus. It is characterized by redness, swelling, and tenderness of the eyelid. A STY is an inflammation of the oil glands in the eyelid, which can cause redness, swelling, and pain. Both conditions can be treated with antibiotics or warm compresses.