Aaaaaactually ~ I believe the real reason one should not do yoga whilst menstruating is this:

In yoga, your menstrual, or "moon" cycle, is considered to be part of apana, or "outward flow" of energy, when the blood that once lined your uterus is being released so the cycle of fertility can begin again. Inversions are poses where your pelvis is higher than your heart, which can be excellent for your circulation. "However, when you are in your moon, inversions are going against the flow due to the effect of gravity on the blood vessels of the uterus," says Shiva Rea, a Yoga Journal contributing editor who teaches at Yoga Works in Santa Monica, California. "Your menstrual flow may lessen for a brief period before resuming its rhythm."

http://www.yogajournal.com/../../practice/548.cfm

so there you have it. stay away from the downward facing dog when you are during your howling-at-the-moon-cycle.

I am LOVING your blog. damn you're a cute girl! and you remind me so much of, well, me. I just had to go back a few years and see how this all began...

keep on rockin in the free world, bay-bay.