Women get upset for a number of reasons and her menstrual cycle isn't always to blame for her anger or moodiness. There are many different symptoms of PMS and not every woman suffers from them. If she's pissed, chances are that something actually caused her anger besides her disgruntled uterus. You may think you're helping by suggesting that her mood swings are out of her control, but you're making things worse. Making her period the scapegoat for her emotions also suggests that she has no self-control and isn't thinking or behaving rationally. And we shouldn't have to tell you that's a bad look.
Blame her emotions on her period