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, if you decide to use a slower shutter speed (say, 1/30 sec.), this means that you have to compensate by closing down the aperture to allow less light in. It makes sense really. Since you have increased the time the shutter remains open to allow light in, you must compensate by allowing less light in to expose the image sensor in that longer amount of time, if you still want a properly exposed picture.