Should I fast and pray tomorrow after the fortieth day, knowing that the blood returned to a dark red color on the 35th day of postpartum bleeding and has not stopped yet, and my monthly period is irregular?

This blood that flows after forty days: if it coincides with the time of your regular period, then it is menses (hayd). If it does not coincide with the time of your regular period, then it is irregular bleeding (istihada). Therefore, as long as your menstruation is irregular, and you do not know that this blood exceeding forty days coincides with your regular period, then it is irregular bleeding. In this case, you must do what is required of a woman experiencing irregular bleeding: take precautions, perform ablution for each prayer after its time has begun, and with that ablution, you may pray the obligatory prayer and as many supererogatory prayers as you wish. You may also fast, and you are not required to make up that fast.