Twitching or fasciculation is usually a local phenomenon caused by involuntary, uncontrolled contraction of a muscle (or a group of muscles) supplied by a single motor nerve. It is most often a benign condition with an unknown cause. Rarely, it may be a symptom of an underlying neurologic, muscular, metabolic, endocrinal or other disorder.
If it occurs in isolation without any accompanying symptom (weakness, change in sensation etc.), it can be ignored and should pass off in some time. If persistent and bothersome, you may contact a neurologist who can examine you and advise appropriately.