It's because your taking really good hits. When you inhale the smoke deeply into your lungs it can make you either throw up or feel like it. The only time I feel like this is when I smoke from a bong because the hits are much more, for lack of a better word, "intense" than pipes and most other methods of smoking.

If you feel like your going to throw up, chances are that if your smoking it right and not swallowing the smoke that you're going to get high more easily.