Muscle twitching of "non-pathological" cause is called myokymia - it's most likely that this is what you have. Sometimes people get this if they're under stress - the related hyperventilation alters the pH of the blood, which reduced the amount of soluble calcium ions - this makes muscles more likely to spontaneously contract.
You might want to see your general practitioner - if you have any other symptoms or signs, it might be worthwhile checking your electrolyte levels, but this is likely to be unnecessary. If the condition is disfiguring, botulinum toxin injections are sometimes used.