Facial swelling can be caused by both minor and major medical conditions. Many causes are easily treatable. However, some are severe and require immediate medical attention. Common causes of facial swelling include: 1  allergic reaction. 2  eye infection (such as conjunctivitis). 3  surgery. 4  side effect of medication. 5  cellulitis (bacterial infection of the skin of face). 6  sinusitis. 7  hormonal disturbance (such as thyroid diseases). 8  sty. 9  abscess.