Based on my experience, it means you are slightly deficient in magnesium. Magnesium and calcium work together to handle nerve signaling where magnesium is the chemical 'off' switch. If you get low, which can happen from intense workouts, you have a deficiency of 'off' switches and sometimes calcium gets in there creating an unwanted 'on' signal and you twitch.
Try taking a magnesium supplement for a week, or eat a few handfuls of cashews every day for a week (high in magnesium) and it will stop if that is the cause.
I think I read that about 60% of Americans are deficient (per the USDA) and if you drink coffee and workout, then you have two more strikes against you having sufficient magnesium.
The best magnesium supplement is Albion labs chelated magnesium, which is sold at iherb.com under the Dr. Best brand.
I also recall a study that showed that lifters taking magnesium had better avg gains than those that didnt supplement. Unfortunately, I did not file it away so I don't have a ready link, but I did read it.
Anyway, I have had to supplement magnesium off and on since I did bodybuilding in college. As long as I take the supplements, I never get any twitching.
Hope that helps.