At some point in history, Jewish women used to have very regularly scheduled Monthly cycles, and used to have some sensation that accompanied the onset of menstruation. Both of these play important roles in Halacha. The monthly cycle establishes what is called a וסת קבוע, which affects what days a woman has to take into account that her menstruation is about to begin. The sensation (הרגשה) is a Biblical prerequisite the absence of which prevents blood from creating ritual impurity, according to most opinions.
At some point in history, women stopped having both of these. The earliest discussion of the cessation of הרגשה that I have seen is in the שב יעקב, circa 1750. The Aruch HaShulchan (early 20th century) asserts that women still have הרגשה, they just don't know what it is well enough to recognize it, but this is just a different way in which it was lost (it became undetected).
Rav Yaakov Kaminetzky reported that when he was in Europe most women had regular Monthly cycles. Today, it is very rare, although apparently not unheard of, from what I have been told.
My question is - is there any Rabbinic speculation about why these changes, with significant Halachic ramifications and/or implications, occured when they did? Are they associated with any world events? Is there any metaphysical significance attributed to the changes?