Whatever it is, it isn't fat burning off. Maybe try eating something else before you ride, what you are currently eating may simply be disagreeing with you. Also, you might try not eating for an hour before you ride. Exercise, especially when intense, can upset some people's stomachs. Heck, for some people exercise can cause acid reflux. http://www.lifescript.com/diet-fitne...id_reflux.aspx (Though I wouldn't presume to know if that is what is going on with you.)