All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
There is no sin on you regarding what you did and there is no expiation on you. A woman is permitted to make the intention of fasting conditioned on being pure from menstruation if she has menses that end around that time, and then if she realizes that she became pure before the Fajr, she completes her fast, and she is not obliged to do anything, and her fast is valid.
Shaykh Ibn Taymiyyah held that her fast is valid in any case if she realizes that she became pure before the Fajr because she did what she was able to do.
In any case, you have to make up the fast of that day as long as you did not become pure before the Fajr. You are not required to do anything except for making up the fast of that day.
Allah knows best.