Facial twitch: I believe these twitches are similar to the twitches people get in their eyelids. Usually it goes away within 6 weeks or so. There are other causes such as caffeine, stress, alcohol, electrolyte imbalances and excessive fatigue. Feel better.
A shutter speed of 1/30 for example will let in twice as much light as a speed of 1/60 since it is two times slower. When you decrease the shutter speed by a factor of two you are increasing the amount of light by one stop. Remember that fractions get larger as the number on the bottom gets smaller.
Well, they also apply to shutter speed, but in a slightly different way. By halving the time of exposure, you're letting in half the amount of light (so if you go from 1/2000 sec to 1/4000 sec, then you're letting in half the light). The opposite is true for doubling the exposure time.