I would add that fainting happens for the reason of certain parts of the brain not getting enough blood circulation. And that can be caused by lots of factors, including:
– low blood CO2 level
– low blood pressure
– too low or too high blood sugar level
– brain vasospasm
– strong emotions
– a state of shock, etc.
I think most probable freason in your case is the combination of low blood CO2 level and maybe low blood sugar / blood pressure level. Make sure you’re not too tired or hungry (or just dined) while you’re practicing, and everything should be fine. Also, always choose the right training complexity for your patterns: http://pranabreath.info/wiki/index.php?title=Training_complexity