Praise be to Allah.
It is not permissible for a menstruating woman to hasten to do ghusl and pray and fast until she is certain that her period has ended.
A woman may know that her period has ended from the emission of a white discharge which is known to women. Some women know that their period has ended when the blood stops.
So a woman should not do ghusl until she is certain that the menses has stopped.
Imam al-Bukhaari (may Allaah have mercy on him) said:
“Chapter on the beginning and end of menses; the women used to send to ‘Aa’ishah pieces of cloth on which were traces of yellow, and she would say: ‘Do not hasten until you see the white discharge,’ meaning thereby purification from menses. Bint Zayd ibn Thaabit heard that the women were calling for lamps in the middle of the night to see whether their periods had ended, and she said: ‘Women never used to do that,’ and she criticized them.” End quote.
Pieces of cotton refers to pieces of cloth which the women would use to check whether the period had ended. The white discharge refers to if they used the piece of cotton and it came away clean with no trace of yellow.
Al-Haafiz ibn Hajar said:
The scholars are agreed that the onset of menses is known from the emission of blood at the time when menstruation is possible, but they differed with regard to the end of menses. It was said that it is known by the cessation of bleeding, which is when something inserted into the vagina comes out dry; or it was said that it is when the white discharge appears – this was the view favoured by al-Bukhaari.
This also indicates that the white discharge is a sign of the end of the menses and the onset of the period of purity (tahaarah). The view that it is known from the cessation of bleeding is countered by the fact that a cloth may come out dry during the period and that does not mean that the menses is over. This is unlike the white discharge which is emitted by the uterus when the menses stops. Maalik said: I asked women about that and it is something that is known to them, which they recognize at the end of the menses.
Fath al-Baari, 1/420
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: If a woman’s period ends before Fajr and she does ghusl after, what is the ruling?
Her fast is valid if she was certain that her period had ended before dawn came. What matters is that the woman is certain that she has become pure (i.e., that her period has ended). Because some women think that they have become pure when they have not. Hence the women used to bring their cotton pads to ‘Aa’ishah (may Allaah be pleased with her) and ask them whether the sign of purity was there, and she would say to them: “Do not be hasty until you see the white discharge.”
Women have to wait until they are certain that the menses has ended. If a woman has become pure, then she should form the intention to fast even if she does not do ghusl until after dawn comes. But she should also pay attention to the prayer and hasten to do ghusl so that she can pray Fajr on time.
Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 17/question no. 53
The questioner did ghusl at a time when she was not certain that her period had ended, and she discovered later on that she was in fact pure, after sunset according to her own account.
Based on this, what she did was not correct, and her fast on that day was not valid, so she has to make up that day.
We ask Allaah to grant her beneficial knowledge and enable her to do righteous deeds.
And Allaah knows best.