The etiology, severity, and cause for concern in regards to a sudden muscle twitch or jerk depends on which muscle is experiencing this, how often is this occurring, what is the duration of the "twitch", and is there pain or discomfort associated with it. I will assume the twitch is only an occasional event of a few seconds duration with no pain involved. In this case, the cause could be from a number of factors like over consumption of caffeine, mild electrolyte disturbances, or even emotional stress.
Generally speaking, when a muscle twitches the underlying problem is not a muscular one, rather it has a neurological origin like a mild neuritis of the nerve that innervates the particular muscle that is twitching. This could be due to an over stimulation of the nerve, an inflammatory process, hypocalcemia, hypokalemia, hyponatremia, or a B vitamin deficiency. This could also be due to a more serious underlying neuromuscular disease.
I encourage you to consult with your physician if the frequency or duration increases or if pain is experienced with the movement.
Sudden jerking, or twitching of muscles can be related to multiple things. When a muscle begins twitching it is because the muscle is contracting. Usually, minor and occasional muscle twitching is common. For example, perhaps an eyelid begins twitching but seconds or minutes (sometimes even a couple of days) after it stops. Other times (and more rarely), it could be a sign of a nervous system disorder.
Minor muscle twitching could be caused by a deficiency in the diet, too much caffeine, certain medications and stress or anxiety. Some more serious causes of muscle twitching are Lou Gehrig's Disease, Muscular Dystrophy or nerve damage. A doctor should be seen right away if the twitching is accompanied by loss of muscle mass, muscle weakness, and loss of feeling.