I'm a fainter, but only during sex. It's a rather common thing for me. And I think it's the result of being overwhelmed by pleasure, your pulse quickening, your breathing quickening and getting shallow, and not enough air making it to your brain. That's what I really think it is. I'm an asthmatic, so it's a bit more severe with me, but I take medication daily to prevent attacks, and have never had one during sex or a scene. Just relax.. if you pass out, it's your body's way of getting oxygen to your brain, and it's a huge ego-boost for your sweetie. You'll be totally fine, and totally safe.

If, however, you do feel it's a concern, see your doctor.