I say, and with God's help: The principle of menstruation is from the time the blood exits to the external part of the body. If it does not reach the external part due to the cotton (kursof) that women use during menstruation, the prayer is not interrupted. The exit is only confirmed when the blood reaches what aligns with the external part from the cotton. If what aligns with the internal part turns red from the cotton, the exit is not confirmed unless the cotton is removed, and the exit is confirmed from the time of removal. This ruling applies to irregular bleeding (istihada), postpartum bleeding (nifas), and urine. See: Al-Muhit Al-Burhani, pages 433-434, and God knows best.