She was not certain that her period had ended, and she prayed and fasted

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? 

He replied: 

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.