A woman must refrain from all forms of transmutation during her cycle. This includes not just sexual transmutation, but also Pranayama, and any form of vocalization. During this time the feminine body is cleaning itself, and if a woman practices a transmutation exercise, this waste can be transformed back into her body which can only lead to nefarious results.
During the heaviest days of her menstrual cycle, a woman should not take baths or showers; she should avoid getting the feet and head wet because the chakras of the head and feet absorb the dampness into the body and damage the uterus during this time. Instead take sponge baths.
Meditation, self-observation, retrospective analysis, etc., are all excellent and recommended to practice during this time.
There also persist false notions that the menstruation is a form of fornication. This is absolutely untrue; menstruation is a natural and necessary function of the feminine organism.
There are numerous groups who teach that sexual transmutation should be practiced while the woman is menstruating. This is a practice of Black Tantra. Such techniques infect the man and the woman with the waste products that the woman's body should otherwise be cleansing itself of. This results in grave damage to the spiritual, psychological, and physiological health of the man and woman. Such practices are rejected in all forms of White Tantra.