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.
Camera aperture and shutter-speed controls are calculated so that an increase of one f/number compensates for a one-step decrease in shutter speed. For example, 1/250 at f/8 is equ*alent to 1/125 at f/11. The table below shows how aperture and shutterspeed are interrelated. All the combinations give the same exposure.