I observed five out of the six fasts of Shawwaal in the month of Shawwaal. Unaware that the month of Thul-Qi'dah had begun, I observed the sixth fast on the first day of Thul-Qi'dah. Will that sixth fast count? In the same context, on one of the days I fasted in Shawwaal, I detected the blood of menstruation an hour after sunset, but I did not know whether or not it flowed from me a short time before breaking the fast at sunset or not: would this day be count as one of the six fasts of Shawwaal?
All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If you delayed the sixth fast until the month of Shawwaal elapsed with a valid excuse, then we hope that Allaah The Almighty will give you the reward of the six fasts of Shawwaal as if you fasted them all in Shawwaal.
Shaykh Ibn ‘Uthaymeen said, "If one delays, without valid excuse, making up for the missed fasts (of Ramadan) beyond the month of Shawwaal, and then followed them by the six fasts of Shawwaal, he/she would not receive the reward assigned to the six fasts of Shawwaal, since the Prophet, sallallaahu ‘alayhi wa salam, restricted it with his statement: ‘And follows it (i.e. the fasting of Ramadan) with six fasts of Shawwaal…’ But the ruling would be different if one delays making up for the missed Ramadan fasts for a valid excuse, such as the case of a woman who is having her post-partum bleeding in Ramadan, which ends in Shawwaal, so she begins to make up for the missed Ramadan fasts and cannot finish from them except after the end of Shawwaal. Then, if she observes the six fasts of Shawwaal after these fasts, she would get their reward because she delayed them for a valid excuse."
As for the blood of menstruation detected by the questioner sister an hour after sunset; if you did not know that this blood had flowed before sunset, then your fasting is valid, for the basic rule is that you were clean and your fasting was valid. Fasting would not be nullified because of mere doubt that the blood might have flowed before sunset.
Allaah Knows best.
You can search for fatwa through many choices