It's kind of hard to keep track of unless you are very aware of her menstrual cycle. PMS symptoms usually occur during ovulation (two weeks before her period starts) and/or starting a couple of days before her period comes.
It has to do with rapidly shifting hormones, and don't worry: she feels just as miserable as she is making you feel, and probably even more so.
I suggest you keep track of her extreme moodiness and the first day her period starts by marking them (discretely) on your calendar. (Don't tell her unless you want her to chop of your penis while you sleep.) You should be able to see a pattern after a few months if her menstrual cycle is regular. If it isn't regular, good luck. You'll never be able to figure it out.