I also wonder if it's the way I'm dealing with the stress of pushing my limits? When I get to a certain point of being uncomfortable the feeling of yuck kicks in. My way of releasing the stress.
I would guess that's not as impossible as one may think. I think I remember you taking part in the discussion of the mental game some time ago, have you worked on relaxation techniques or visualization? If you find yourself very nervous pre-race or during the race, this could affect your stomach and maybe those techniques could help take an edge off the nerves.
I remember going through a period where I was so nervous before races that I would feel a bit nauseous even before the race started. Needless to say, the nerves were bad enough to affect my race performances.