One of my clients has been experiencing some nausea during her workouts. She isn't on any medications, she is not pregnant, and she has a good diet. What could possibly be causing her to experience this nausea during her workouts?
There seems to be two schools of thought on this. One is that this nausea is the result of hypoglycemia. Obviously, hard training burns a great deal of glycogen. You mention that your client has a good diet, but ask her specifically about her pre-workout diet. She may be exercising on an empty stomach. Ask her to experiment with different pre-workout snacks to see what works best for her. Be careful with sports drinks, however, as some of them can cause gastric discomfort.
The other school of thought indicates that nausea is indicative of the release of growth hormone. Arnold Schwarzenegger stated in one of his books that he and his workout partners always kept a bucket close by during their heavy squat workouts. To them, a "bucket workout" meant a productive workout.