Is it necessary to repeat the fast of two days during Ramadan in which a small amount of blood descended after 'Isha, then stopped after taking birth control pills to prevent menstruation?

If you intended to fast from the night and did not see blood until during the day, then your fast is valid. This blood, whether it is menstruation or not, does not affect the validity of the fast, because purity within the menstrual period is a valid state of purity. This blood is considered menstruation by the majority of scholars if its total duration reaches one day and one night and it occurs within a time frame suitable for menstruation.