It depends on how close you are, I suppose. I think it's coming from a good place. My twin sister is normally the sweetest person, but around her period she becomes highly paranoid and belligerent. I have been sucked into many arguments where halfway through I asked, "Wait, are you PMSing?" and she would realize she was and the conversation would end in giggles. We are very close and since her hormones have been such an issue I know she wouldn't mind if I went the extra mile and tracked it on a calendar.

It seems like she is trying to understand your symptoms. I think it's nice that she cares so much, but if you feel it's intrusive you should tell her to leave you alone.