Apart from eating too soon before very hard riding, the only other times I've felt like throwing up is when I haven't warmed up properly.
Classic scenario: you drive to a race, get there a little too late, don't get a proper warm-up, then the pace is hot on the first lap -- blah! Unpleasant. If my HR goes straight up to 185 without a warm-up, I usually get an unpleasant feeling in my chest and stomach which can take more than 20 mins to go away.