Is this period considered menstruation or istihadah? And has the menstrual period become 13 days, or should one purify oneself and pray during the first period of blood cessation?

If the bleeding after postpartum (nifas) coincides with your regular menstrual period, then it is considered menstrual blood (hayd). If it does not coincide with it, then it is considered abnormal uterine bleeding (istihada). The period of purity between the two bleeding episodes is a state of purity during which prayer should not be abandoned, and if it was abandoned, it must be made up. The repetition of this situation indicates that your regular period has changed to eight days, which are dispersed according to the principle of "talfiq" (combining different scholarly opinions), and the period between the two bleeding episodes is a state of purity, not menstruation.