It's because of hormones. They can't control it. It's call PMS. If a girl starts to become very temperamental during her period, just leave her alone. She just wants to be alone. Or try to be nice to her. Go out and buy her flowers or make her a cup of coffee. It's a small act but it can go a long way. And if she tells you to go away, just let her be. Sometimes girls want to be alone during that time, and sometimes they want all the company they can get. It's extremely hard to control it. Remember that it's not her fault and that if she gets mad at you, it won't last long at all. Don't blame her for anything she says because she's barely herself when that happens unless she's learnt to control it. There are some methods but each girl has her own so she's most likely figured some way to ease the emotion by herself.