Why does my girlfriend randomly get moody, not tell me why, and then pretend nothing happened?

My girlfriend got really moody yesterday, seemed depressed, would not tell me what was going on. Today she was a huge bitch. Then, all of the sudden, she feels better and says she can't wait to see me. Still won't tell me what happened.

Is this normal, should I just accept that women have random mood swings and just let it go? This isn't the first or second time this sort of thing has happened.
Why does my girlfriend randomly get moody, not tell me why, and then pretend nothing happened?
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