Fantasy is a mental image of objects, people or scenarios, which is not present. Some experts distinguish between conscious fantasies, and daydreams, or unconscious fantasies. In Freudian psychoanalysis, fantasy is a means of realizing wishes or desires.
Everyone has the potential to be jealous. Some people will experience small bouts of it while it will become an obsession with others. Slight incidents are usually no reason to worry, and you can probably pass them off.