I've wondered the same thing, and am skeptical about "toxins". What toxins? Certainly, the massage isn't causing rhabdomyolysis and myoglobinemia, and lactic acid shouldn't be accumulating.
The short answer is that the whole "toxin" thing is a myth. To explain your symptoms, the article I have linked below posits the following:
"There remains a consideration that some clients respond to a massage with reactions of flu-like aches and malaise. Such symptoms have often been attributed to the toxins released. Dispelling with the concept of toxins means that we must seek other explanations for such post-massage malaise. ...the inability to produce an adequate cortisol response to a stress can result in symptoms: 'Deficiency in cortisol is characterised by fatigue, weakness, muscle and joint pain, bowel symptoms, nausea, increased allergic reactions, mood disturbance'. I tend to think of a body's neurochemical system on the edge of its ability to adapt being pushed temporarily beyond the edge by accommodating to the work being done. This reaction may be exacerbated by effects of athletic overtraining or by a genetic metabolic predisposition"
Here is the full article:
http://www.massagetoday.com/archives/2002/12/08.html
Here is an answer from another site that goes into great detail and provides references:
http://answers.google.com/answers/threadview/id/602783.html
I thought it better to reference it than steal it
Biochemically and physiologically, the toxin thing just doesn't work. Something is going on, but it's far more complicated than "flushing toxins".
Thanks for bringing this up - it's fascinating, and something I was curious about too.