Women are allowed to perform Umrah in menstruation. It was narrated in the Hadiths that the Prophet (pbuh) ordered Aisha (RA) when she was menstruating to take a bath and assume Ihram. He also commanded Asma Bint Umays after she gave birth to a child, to clean herself and wear pads, and then to enter the state of Ihram. Therefore, it is quite clear that women can assume Ihram during menstruation or postnatal bleeding after cleaning herself and perform the rituals like other pilgrims except Tawaf around the Kaabah.
If a woman is in her menses while leaving home, she should clean herself and take a bath (ghusl) or if it is harmful, she should make wudu and then assume the state of Ihram. Then while facing the Qibla, she should form intention of performing Umrah and utter Talbiyah. However, a woman in her menses should not offer the Nafl salah after assuming Ihram. If the menses begin after assuming Ihram, the Ihram remains intact. Menses does not disturb the state of Ihram.
However, such a woman should remain in her hotel or apartment in Makkah and should not enter Masjid-ul-Haram till she takes the bath of purification. She should continue to recite Talbiyah, Takbir, Tahleel, Istighfar but avoid reciting the verses of the Holy Qur’an. She can perform Umrah after taking the bath of purification.
If menses begins while performing Tawaf, the woman should stop the Tawaf and come out of Masjid-ul-Haram. As Saa’ee follows Tawaf, she should also refrain from performing Saa’ee. She can perform Tawaf and saa’ee after becoming clean. If menses begins just after performing Tawaf, she can perform sa’ey which is permissible in this condition.
There is nothing wrong in taking pills in Umrah to stop menstruation or using tablets that prevent menstruation as long as it does not affect ones health or is not prohibited in Islam. If a woman wants to take contraceptive pills to stop periods during Umrah so that she would be able to perform Tawaf or fast during Ramadan, there is nothing wrong in doing so.
Note: It is recommended to consult and seek an opinion from scholars on doubtful Islamic matters.