During menstruation, a lady’s body normally discharges blood from the uterus. This cycle stays for an average 6-7 days each month. During this period, she is not allowed near holy places. Reason being she is termed impure.
Ironically, as per logic, this cycle makes her very pure — far from being impure. Men don’t have this cleaning phase.
So during her menstrual cycle if she enters a temple (as per south Indian tradition), the energy present inside the temple moves to the purest place, i.e. into her body, and stays there. When the energy moves to this pure place, i.e. her body, positive vibrations in that particular temple shifts. To restore that energy, tantrik and other high priests have to purify the temple.
This is also the reason why, in south India, new born babies are not allowed inside temples. The energy shift happens.
You might observe that, during menstruation, if a lady touches the Tulsi plant (a holy plant), the plant slowly dries out or dies because of the energy shift.
To avoid this commotion, the easiest way was to put menstruating ladies in bad light.
During my grandmother’s time, they were made to live in a separate room, cook by themselves, not talk with anyone, avoid looking at anyone and made to feel inferior. In some parts of north & south India, this cruel process is still followed.
Times have greatly improved. I used to feel angry during my 20s at women being denied entry into temples (women in other faiths had no such rules). It was then that I started reading and understood the logic.