Why is it usual to be nervous and sometimes feel sick or properly scared before a running race? (eg. 100m) ?

It is a reflexive reaction due to an increase in adrenaline in the blood stream due to the "flight or fight response" either to run for your life or fight for your life. Adrenaline gives you a surge in energy, keeps you from feeling pain and allows you to compete better. This happens to everyone, even Olympic champions, before a race. While it will never go away, as you become more accustomed to it, it won't be as bothersome.