Everyone who takes running seriously gets nervous before a race, and you will find that several of the top runners throw up regularly beforehand. If you are not nervous then the chances are that you will not be mentally strong and will perform badly.
You will also find if you ask that most people feel ill, injured, tired and in no fit state to run well. That usually means they will do a p.b. Perhaps your body is just preparing itself for the ordeal by focussing your attention on it.
Take it as a good sign.
Hmmm explains the queues at the portaloos too. Top tip for the nervous runners, take loo roll, wipes etc in a little plastic bag to start area, oh and a paper to read once you've exercised squatters rights on your chosen accommodtion.