So, you think its PMS and nothing deeper. However, then go on to say that she basically only has this attitude with you; not her co-workers, family or friends. There is a significant contradiction in your statement if her actions are not congruent throughout her interactions with all people.
Additionally, if it is just PMS, does she also exhibit other behaviors during this phase which are congruent with your hypothesis? Does it also occur at the same time in her cycle each month? Can you control her cycle? Can you control how you react to her cycle?
I believe that using her biology as the only source of her actions is a disservice to the relationship. You should always look at ways to improve your interactions with others, especially those you love and cherish. By relegating her actions to PMS and subsequently expressing that to her, it is an illustration to her that you may not have the capacity to handle difficult emotional situations, this is a possible
message she may recieve from your actions. Now I am merely throwing out ideas based on the data you have provided, not suggesting you are incapable of such behaviors.
This happens quite often with men, especially those on this website. Many are incapable of handling their own emotions, which I am guilty of as well, however, I will certainly admit to that. Distance and avoidance of her emotions is not a healthy solution for LTRs, it will build disconnect and resentment, which often comes shortly before broken communication and termination of the relationship.
So with that, here are parting words...
1. Dont EVER tell a woman it's because of her PMS or "time of the month". You may know it, but keep it to yourself. I am sure all of the women here would agree on this
2. consider this... you have been responding the similarly to her cycle every month, but the problem persists. That is the definition of insanity, doing the same thing over and over and expecting a different result.