The only time I ever threw up from exertion the circumstancees were similar to yours. I had had a coke for breakfast, then I had to tred water for ten minutes. It might not sound like much but I'm very negatively buoyant and very skinny, so it was akin to an all out sprint for me. I got out of the water and was standing around with my friends when I just started feeling very bad. I lost it on the deck and felt better instantly.
Don't eat directly before your workout either. An hour or two to let the food get digested is a great idea. I've regretted a few hamburgers that got eaten right before rides.
I guess the lesson here is that sometimes really obvious things (don't eat too much and don't starve) are actually good ideas.