Stomache problem

[mkgehrin@n-jcenter.com]

About midway through my first two rides (a 35 & 25) my stomache starts
feeling really gross and burpy.  By the time the ride is over, I have to
sit down every few minutes because I feel like I'm going to throw up.  I
never actually do throw up but that would be preferable.  Anyway my
stomache feels nasty for the next day or so.  Other than that, my horse
and I are fine.  One ride was cold and wet, the other ride was hot so I
don't think its weather related.  I didn't eat or drink much the first
ride so I drank and ate more the next.  Same gross stomache.  I had
cereal with wilk about an hour and a half pre-ride.  Is that the
culprit?  Did it churn up into butter during the ride?  I had fruit
juice, crackers, skittles and an apple during the ride.  I would greatly
appreciate any input. 

Still sick from Florida Endurance Classic,
Karen in Ormond Beach