mood swings

Are all women like this? One time she's happy-go-lucky, another time she doesn't even want to talk to me. How do I know if it's because of something I've done that's offended her or it's just pms? Why does she say just "fine" when i ask her how she is and "nothing" when I ask her what's the matter? Does any man out there truly understand women and their mood swings?