Brief left side of face numbness and lower left arm numbness that went away after a few seconds. what's wrong?

Numbness: It is probably nothing unless it happens again at your age. Did you have a headache? If so, could have been a migraine. If it continues see your family doctor. They will work it up for you. Chances are at your age it is nothing. Good luck and feel better.

Parasthesias: These symptoms are called parasthesias and could be a result of a number of different issues including anxiety or panic attacks, malnutrition, vitamin deficiencies, neuropathies or nerve disorders, vascular problems, inflammatory or infectious diseases. This is only a short list, there are many causes of numbness and more history and an exam would need to be done to confirm a diagnosis or cause.