The cycling club I ride with likes to ride *very* fast sometimes. Some rides though, I find myself so exhausted that I get the dry heaves. So, I slow my pace a little, recover a little, then try to push hard again, and get the heaves and have to slow down again. We leave on the rides at 630am, and I can't eat before riding. I make sure I eat a good dinner the night before for fuel, and drink a good bit of water when I first wake up. Back when I was a competitive swimmer the same thing would happen - I'd barf after an event if I ate even 3 hours before I competed.

What causes this???