All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The answer to this question is based on the knowledge of the minimum period of purity between the two menses. The majority of the scholars are of the view that it is 15 days.
Therefore, if a woman sees blood for nine days, for example, and then she becomes pure for a period of less than 15 days, and then she sees blood again, then this is considered Istihaadah (bleeding outside the monthly menses).
The Hanbali School of jurisprudence is of the view that the minimum period of purity is 13 days.
The view we adopt here in Islamweb is the view of the Hanbali School of jurisprudence.
Therefore, if a woman sees bleeding after 13 days or more, then this bleeding that she had again is menstruation. If she sees bleeding after less than 13 days, then it is Istihaadah. There is no problem if the total period (of bleeding) that she had in a month is more than 15 days as long as the period of interruption between the two bleedings lasts for the minimum period of purity between two menses.
Allah knows best.