All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
If the questioner wasn`t certain that she had discharged menstrual blood for a second time in the same month then, in principle, she is ritually pure because certainty may not be disproved by doubt. Therefore, she is obliged to make up for the days in which she had her menses.
Ash-Shirbini (May Allah be pleased with him) said: "Whoever is certain about being in state of purity or impurity, but had doubt about any then s/he should act on what s/he is most certain about because Allah’s Messenger (PBUH) said: “If one of you feels disturbance in his stomach and doubts whether he has released some wind or not, then he should not leave the mosque unless he hears its sound or smells (its) odour”. [Reported by Muslim]." {Moghni Al-Mohtajj}. And Allah Knows Best.