Is the blood that comes down ten days after the end of the menstrual period, and is accompanied by menstrual pains, though in a smaller quantity than usual, considered menstrual blood or istihadah (non-menstrual vaginal bleeding)?

The apparent conclusion is that the aforementioned blood is istiḥāḍah (non-menstrual vaginal bleeding), because the minimum period of purity (ṭuhr) is fifteen days according to the majority of scholars. Therefore, any blood that appears after the end of the regular menstrual period and after verifying purity is considered irregular bleeding (dam fasād), unless there is a period of at least fifteen days separating the two bleeding episodes. You should continue to pray and fast.