Praise be to Allah.
If a menstruating woman becomes junub, or she is junub and her period begins, it is prescribed for her to do ghusl to cleanse herself from janaabah. The point here is that she will then be able to recite Qur’aan without touching the Mus-haf, because the one who is junub is not allowed to touch the Qur’aan, unlike one who is menstruating. See question no. 2564 and 60213.
The way in which this ghusl is done, as in all other cases prescribed by sharee’ah, is to start by washing the parts of the body that are washed in wudoo’, then pour three handfuls of water over the head, then wash the right side then the left, then pour water over the whole of the body. Thus janaabah is removed but menstruation remains.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (1/134): If she does ghusl from janaabah at the time when she is menstruating, her ghusl is valid and the ruling on janaabah no longer applies to her. This was stated by Ahmad who said: The janaabah is removed but the menses is not, until the flow of blood ceases. He said: I do not know of anyone who said that she does not have to do ghusl except ‘Ata’, but it was also narrated that he said that she should do ghusl. End quote.
And Allaah knows best.