What Causes Eye Twitching?

Hello- I would like to ask what makes your eyes twitch? My left eye has been twitching on and off for a week. It is not painful, but can be irritating. There are no other symptoms. I am a 56 year old female, only med I take is Premarin .45mg daily. I take multivitamin and multimineral daily as well. Will have eyes examined in March. Do use OTC eye drops as prescribed for dry eye. Thank you for any feedback.