Lactic acid doesn't build up in the stomach, it is part of the fuel system that helps muscles drive when there isn't enough oxygen being delivered through the bloodstream. More likely that you pushed your body into stress, and that has a variety of outcomes from nausea, to vomiting, hyperventilation, muscle spasms, all sorts of wierd and wonderful things. The best way to avoid it is to experience it on a regular basis!

I'm not kidding, just keep building your fitness up and start getting hard intervals into your training regime. Your body will adapt over time and you will be able to cope with more stress during a race before the symptoms come on (if at all).