Last week upper left eyelid started twitching 1 or 2 times a day. Getting more frequent. Not more stressed or sleep deprived or drinking more caffeine than usual. What else could it be?

Nothing serious. Just a benign muscle twitch. Try drinking more water, and add a magnesium supplement daily. 500mg a day should do the trick in a week or so.
Myokymia. Most often eyelid twitching is due to muscular irritabiity termed myokymia which is annoying but quite benign. I may be associated with eye strain, fatigue, or stressors, but may occur without any clearcut etiology. It the twitching involves the facial muscles in addition, best to have this evaluated.