I was going to suggest sodium but Mel beat me to it.

Nausea is the #1 symptom I get when I'm not getting enough sodium. For me, though, it persists throughout the day (and will go on and get worse for as many days as it takes for me to figure it out and drop a few Zenergize tablets or whatever ), so I'd still wonder in your case whether it wasn't simple overheating.

I know people who keep water bottles in the freezer, so they'll thaw and provide cool water as they ride.