Women can be cranky when they have their periods. Then again, would you be happy if you spent five days feeling like you've been kicked in the stomach? Here are some ways to be more supportive and understanding toward your girlfriend when she has her period.
1-Never assume that a girl is in this situation unless you are completely sure, as it is incredibly offensive. Although some men may be sympathetic at 'that time of the month', the general perception that women have is that men can't actually understand what a woman goes through during menstruation, given that they do not experience it. To assume that a girl is in such a circumstance, without being told, is arrogant at best and completely unreasonable at worst.
2-Get informed about PMS. If you don't know the exact reasons for its existence, you may never understand why it happens. This will leave you feeling like your girlfriend is treating you like a doormat for no or close to no reason and might take a toll on your relationship.
3-Realize that you cannot control the situation no matter what you do. Realize also that mood swings in girls in this situation are not necessarily under their control. Knowing this, and understanding that it will happen one way or another is your best strategy.
4-Find out what her usual pattern. Is she usually in pain/depressed/tired? Try to adapt to these patterns.
5-Don't antagonize her. Easier said than done, but avoid piquing her temper as much as possible.
6-Do something "thoughtful" for her. If possible avoid getting something in return: this will prove to her that you want her to be happy, and it's not that you want something from her.
7-Prepare for extreme reactions. This goes in both directions. So if you're nice to her to make her feel good, she may cry tears of joy over some flowers or you doing her dishes. Don't let it freak you out. Hug her (carefully, this is not the time for rib-cracking squeezes), give her a kiss and hand her a tissue. She'll love you for it.
8-Comfort yourself with an understanding that she is not enjoying her period and treat her with tenderness, but with a quiet strength. Your girlfriend will react well to your self assurance.