Nausea after a race?

Many things can cause nausea, including what you ate and a million other factors.  That said, what you are describing is likely attributed to asking your body to do something that it is not used.  Anaerobic Glycolysis.If you are like most Masters swimmers, you are probably doing 3,000 to 5,000 yards of long swimming in a practice.  Moderate intensity is probably the limit.   You are likely training your aerobic system, which is great for your long open water swims.  You are not, however, preparing for the sudden increase in heart rate and build-up of lactic acid experience in a hundred.You are probably not stressing the anaerobic system, like you did 'back in the day'.  So now, after you trained your body to swim at a nice pace forever, you are asking it to go 100% for 50 seconds.  In order to do this, your body uses Anaerobic Glycolysis to produce quick powerful energy.  The byproduct of this is lactic acid build-up in your muscles.  Now for kicker, since you never ask your body to do this, it is not trained to metabolize the lactic acid efficiently.  You are putting it into a state of shock.  Likely causing you to vomit.You mentioned that 30 minutes of warm down relieved the sensation.  You are in essence allowing your body to 'handle' the unexpected lactic acid.The Answer...If you want to compete in hundreds, add some high-intensity training to your workouts.  Your body will adapt and become trained to tolerate it.