Are the first days of menstruation considered istihadah if the period of purity between two menstruations is less than fifteen days?

The minimum period of purity between two menstrual cycles is thirteen days. So, if menstruation ceases and then blood returns before this period has passed, and it is not possible to consider the two instances of bleeding as one menstrual cycle (meaning the duration of menstruation and the purity between them did not exceed fifteen days), then the returning blood is considered istihada (non-menstrual vaginal bleeding). The criterion is not the pain of menstruation, but rather the sighting of blood.