Inversion poses aren't advised because they send the energy in a different direction than your body is going in during menstruation, to sum it up in short.
So, it's more about the flow of energy, about working with your body's natrual flow at that time, not about damage to you. Here's a nice simple piece on it at Yoga Journal (which is an awesome mag, if you don't already get it): http://www.yogajournal.com/practice/1187_1.cfm
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