Now of course men are no better, but we're not talking about men.  Women, at times can get emotional...surprise.  And no, it's not only when they are getting their period, which makes figuring them out all that more difficult.  If you knew exactly when their mood would change, you could prepare yourself for it.  That said, there's really no way to truly prepare for a sudden mood shift.

Maybe it's something you said, maybe it's something you didn't say...or didn't do.  Whatever happened all you know is now she's not talking to you.  She got up and left the room.

When this happens, the important thing is not to follow her like a sorry puppy asking her, 'what's wrong'.  She's pretty pissed as it is, and by asking such an oblivious question, may set her off even more.  Let her cool off. 

If you're the reason you pissed her off (or got her upset), then wait a little while, say 20-30 minutes then go talk with her.  If you believe she got upset out of no fault of your own (which is hardly ever the case), then sure, go talk with her, but don't give into her mood and get all squirmy yourself. 

The point is, you need to be her, 'rock'.  She can't have you all pouty following and sucking up to her every mood swing.  Allow her to go through all her moods and express them as she feels necessary, without getting yourself swept up in it all.  She needs you to be consistent.  Give her that much because God knows she put up with enough of your bullshit...