can i just say...
The absolute last thing you should do when I'm PMSing is ask if I'm PMSing. I don't care how you ask it, or say it, but do not imply that my feelings at that point at that time are irrational- even if they are. Seriously, I've had boyfriends in the past who did this and they had the wrath of god unleashed upon them. Like some other posters have said, the best thing to do is listen to my craziness (you don't necessarily have to agree, but listening and NOT starting a fight at that time is good), sympathize, and offer a back rub, or run a bath or something. Most of the time I'll realize how off-the-wall i was being the next day and say I'm sorry. I don't really agree about the whole backing away slowly thing... usually i get really emotional and NEED someone to listen to me- it'd probably piss me off more if my boyfriend tried to leave me alone.
Can I also point out that if your girl is pissed off at you for whatever, maybe it's a legitimate reason? Blaming every emotional outburst on PMS is not only sexist but undermines legitimate relationship problems.