[trux]

You should try making an intensive session of hypocapnic static training to see if it has diarrheal effects too. At such training, you do not really achieve any deep or prolonged hypoxemia, and do not use muscles, so the lactic acid build is rather limited. In contrary CO2 will be high and you will likely experience strong and long contractions. That should help isolating the culprit of the diarrhea - if such training has the same effects, then the contractions and blood shift are likely at its origin. If it has no or a weak diarrheal effect, then lactic acid may be the reason. In that case testing your liver and resting lactic acid level may be wise to exclude some pathologic factors.

Such a session of about an hour may look like follows:
1) warm up (i.e. 3-4 easy apneas of 1-2 min slowly exhaling underwater until empty lungs)
2) 2-3 moderate empty lung apneas with relatively short recovery (30s)
3) 10 minutes of same length apneas (i.e. 1 min) with only a single exhale/inhale cycle between them. Try keeping the breath-hold length same or longer as you progress, do not decrease the time. So you better start quite easy - it will become hard at the end.
4) Classical hypercapnic table - i.e. apnea of 2 min + rest 1:30, 2 min + 1:15, 2 min + 1:00, 2 min + 0:45, 2 min + 0:30, 2 min + 0:15, 2 min + 0:15, 2min + 0:15, and the last 2 min apnea (increase the 2 min apnea accordingly to your level - usually around 50% of your PB is OK)

Also try comparing dry and wet training - possibly water getting accidentally into your mouth and gastrointestinal tract may be a factor too, or it may be the temperature that play a role too.

You can also try changing your diet - i.e. eating rice, but early in the day, not directly before the training.