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.