WHY DURING MENSTRUATION WOMEN WERE NOT ALLOWED TO DO ANY WORK OR COOK ANY FOOD ITEMS?

The women in the families were required to be kept in quarantine for three days in order to prevent infection (as menstrualtual discharge had some toxic substances) and also to give the women the much needed rest during the period.

This seems like a regressive thought that our ancestors suffered from, but our ancestors kept in mind the discomfort and trouble that a woman goes through while menstruating, suffering from cramps and exhaustion. 

To keep women away from all work and to give her the much-needed rest, this theory of banning a menstruating woman in those days and not allowing her to cook (because cooking was a major activity and the woman had to cook for many members) came into being. Though over the ages, it did become a notoriously backward practice.