Is it necessary to make up for the day on which I drank water, thinking the sun had set, even though I don't recall its definite setting, after having fasted a makeup day from the previous Ramadan?

If a fasting person drinks while in doubt about the غروب (sunset), he must make up that day because the original presumption is the continuation of the day. However, if he drinks with the strong presumption that the غروب (sunset) has occurred, he is not required to make up that day, unless it becomes clear to him that it was otherwise.