Fluttering sensation in leg. What could it be?

Isolated muscle. Twitches or fasciculations are common and usually not serious. It could be from drinking too much coffee, stress, anxiety, electrolyte imbalance etc. However, seek medical help if it's accompanied by increasing pain, weakness, numbness, fatigue, fever, weight loss etc. http://www.livestrong.com/article/86285-causes-muscle-twitch/.