I was feeling naseous when I did "core work". Someone on this site suggested I may not be breathing right. I tried to be consious of that and it did help.

I have almost thrown up when I have had a really intense lifting session. In hard core gyms, like powerlifting and body building gyms, they have "puke buckets"!
Try not to eat just before your workout. No sooner than about an hour before and stay hydrated, but try not to drink too much either,that can make you nauseous as well.

Puking is your body's way of saying "you want me to do WHAT?!?!?!"

Nausea can occur if you push too hard past your current ability. Assuming you don't have any medical conditions that need to be factored in, try backing off on the intensity.

The second time I went to a spin class I was so eager to keep up with what everyone else was doing that I over-did. Got real light headed and just barely made it to the men's room before tossing my cookies and nearly passing out.

I've also found that I am more prone to nausea if I stop too suddenly. Again in spin class I find it helpful to wear a heart rate monitor. If I've been pushing up to 80% or more of my maximum heart rate during the class I keep moving until I cool down to about 100 bpm (my comfort zone) before I dismount. Other wise, well, you know. Bleah.

The same kind of thing can happen when lifting VERY heavy, especially doing something like squats or deadlifts that not only work the legs and back but also tax the cardiovascular system. Power lifters sometimes push themselves so hard that all sorts of bodily functions happen involuntarily. (eeew.)

wrknprogress suggests not eating right before. I sometimes have the opposite problem - if it's been too long since my last meal I'm more prone to light-headedness because essentially I'm driving on empty.

I believe that's a big sign you're working too hard for the fuel your body has to expend. It's always possible that something you ate upset your stomach but not very likely. You could be dehydrated, working at a level that is too high for your ability, working too hard period. I honestly would be evaluated by my doctor to make sure there aren't any problems and then consult a personal trainer at least once or twice so that they can help you with your workout. I've never been sick and if I've even felt a bit sick then I've either stopped or backed off. Best to you.

Also, slowing down your breath (it sounds counterintuitive but it works) when you are exerting heavily can help but please make sure you're not working too hard or that you don't have a health issue that causes this to happen.

Thank you for your responses. I do have a medical condition (lung infarction about 6 months ago) Also, since my blood clot in the lungs my resting bpm is 94 - 103. So when I exercise I monitor my heart rate and dont let it get above 170. I am not even running when it gets up high. Just walking fast on a treadmill (between a 3 and 4). I am under the care of a doctor... no one really knows why im still having problems. Thanks guys