Mensturation is stigmatized in our society. This stigma built up due to traditional beliefs in impurity of menstruating women and our unwillingness to discuss it normally. We don't know what may have been the reason that forced the holy men to refer to menstruating women as 'unclean'. But almost all religions refer to menstruating woman as 'ritually unclean'.
According to hindu beliefs a girl should not visit a temple or offer prayers while she is menstruating. According to these beliefs and customs a girl who has periods is impure and hence, should not even touch anything that shall be given as an offering to God. They also cannot take part in the religious ceremonies. But if logic is taken into consideration it is something that is given by god himself.
Science talks of the menstrual syndrome, i.e., a woman is more irritable during those days because of a hormonal imbalance. Psychics claim that there is a different aura around a menstruating woman.