Must a prayer that was performed after ablution be repeated upon discovering a small spot of blood on the toilet seat, knowing that the underwear is clean, that the menstrual period is near, and that this situation frequently occurs before menstruation due to hormonal imbalance caused by birth control pills?

If you are not certain that a drop of blood has exited, then there is nothing upon you. However, if you are certain of its exit, it is not considered menses by itself, because the minimum duration of menses is one day and one night. If blood reappears after that during the period of menses and its duration reaches one day and one night, then it becomes clear that this drop was indeed menses. In this case, you must make up the prayers you performed during the period of blood cessation, because the purity that intervenes within the menstrual period is a valid purity, and prayer becomes obligatory after performing ghusl during it.