Sheikh `Abd al-Rahmân al-Jar`î, professor at King Khâlid University in Abhâ
You should not make up that day of fasting. This is because there is no way for you to be certain that your menstrual period began before sunset. As long as there is any doubt about when your menstrual period started, you do not go against the previous assumption based on the certainty that you were not menstruating.
You fasted the day upon the certainty that you were not menstruating. You only became certain that you had begun to menstruate when you saw the blood after sunset.
Likewise, the default assumption regarding your fasting for that day is that a fast that you have observed from beginning to end is a valid fast. This validity cannot be overturned by something wherein there is any doubt.
Your fast is intact. The legal principle here is that certainty is never overturned by doubt.
And Allah knows best.
Source: Islam Today