Chalazion is a cyst in the eyelid due to a blocked oil gland. They are typically in the middle of the eyelid, red, and non painful. They tend to come on gradually over a few weeks. A chalazion may occur following a stye or from hardened oils blocking the gland. The blocked gland is usually the meibomian gland but can also be the gland of Zeis. A stye and cellulitis may appear similar. A stye; however, is usually more sudden in onset, painful, and occurs at the edge of the eyelid. Cellulitis is also typically painful.