You make me sick
I wouldn't jump to the "toxins" idea, while metabolic wastes can frequently be stored in soft tissue and be released they are by no means toxic. Plus the theory of flushing out lactic acid has been shown to be a myth, so that can be ruled out too.
Most of the cases I have heard about regarding clients feeling sick after massages relate to the work being to deep, causing mild nausea from the increased circulation to tissues that are normally starved of oxygen and nutrients the blood carries to them. If you do mostly deeper work, like trigger point release, frictioning or muscle stripping and stretching, especially if you spend extra time on their neck, shoulders, or lower back, some of the most common areas for focus work to be on.
Try easing up the work a little bit and slow down the strokes and techniques your currently doing and really emphasize to your clients why good hydration is important for their body to function well. Suggest that they ice the area of focus to ward off a painful inflammation also, because some people can have nausea as a reaction to inflammation.