My thigh has been twitching since last night. Didn't work out recently. What might it be?

See answer below. Isolated muscle twitching can be completely benign or can be associated with various neurological disorders. If it happened once and went away, i wouldn't necessarily worry about it. If on the other hand you notice it more frequently, i would get it checked out by a physician. Most likely, it is a brief muscle cramp from low potassium. Eat more tomatoes and bananas to see if it helps.