Thanks for the input, I hadn't really given the central nervous system much thought & what it goes through when we push the limits.
I did read a few articles on the subject and found a couple of plausible links. One would be low blood sugar, especially with morning races. I deal with low blood sugar on my long runs but never gave it much thought for a short distance run. Could be something there.
Endorphins released during intense exercise was another possible cause of nausea. That also makes sense to me because it occurs when I'm at my most uncomfortable point of racing, when I'm pushing the limits.
I'm running a superset at the track Friday a.m., it's a gut buster workout so I'm going to make sure I have some juice or sports drink before I go to get the blood sugar levels back up. Also, see what my mind can do to keep things under control (or not). I may leave it all at the track.