If a woman makes a mistake about when she becomes pure following menstruation, is she sinning?

Menstruation differs from one woman to another, and the signs
that one woman’s period has ended may vary from time to time. 

For most women the sign that the period has ended is the
emission of the white discharge. For some women the sign is that the
bleeding stops. 

No matter what the sign is for a woman, it is not permissible
for her to hasten until the sign appears, because it is not permissible for
her to pray or fast when she is menstruating, until she becomes pure. 

The women used to send containers to ‘Aa’ishah in which were
the cotton pads with traces of yellow on them. She would say: “Do not hasten
until you see the white discharge.” 

This was narrated by
al-Bukhaari in a mu’allaq report in Kitaab al-Hayd, Baab iqbaal
al-maheed wa idbaarihi (Book of Menses, Chapter on the start and end of
the menstrual flow); and by Maalik, 130   

If a woman makes a
mistake about the time of the end of her period, based on her own reasoning,
then she is not sinning, because Allaah says (interpretation of the
meaning): 

“And there is no sin
on you concerning that in which you made a mistake, except in regard to what
your hearts deliberately intend”

[al-Ahzaab 33:5] 

And the Prophet
(peace and blessings of Allaah be upon him) said: “Allaah has forgiven my
ummah for mistakes, what they forget and what they are forced to do. “
Narrated by Ibn Maajah, 2053; classed as saheeh by al-Albaani in Saheeh
Ibn Maajah. 

But if she thinks that
she has become pure and she prays and fasts, then she realizes that she is
still menstruating, then she has to stop praying and fasting until she
becomes pure, and she should make up the obligatory fasts that she observed
during that time, because it is now apparent that they were not valid,
because the fast of a menstruating women is not valid. 

If she stops praying
because she thinks that she has not yet become pure, then she finds out that
she was pure, then she has to make up those prayers. 

Shaykh Ibn ‘Uthaymeen
(may Allaah have mercy on him) was asked (11/280) about a woman who saw a
brownish discharge before her usual period, so she stopped praying, then the
blood came at the usual time. What is the ruling on that? 

He replied: Umm ‘Atiyyah
(may Allaah be pleased with her) said: “We used to not regard the yellowish
and brownish discharge after the tuhr (white discharge indicating that the
period is over) as being of any significance.” Based on this, this brownish
discharge that comes before the period does not seem to me to be part of the
period, especially since it came before the usual time of menstruation and
there were no other signs of menstruation such as cramps, backache, etc. So
it is better for her to make up the prayers that she missed during this
time. 

He was also asked
(11/275) about a woman who bled for nine days, so she did not pray, thinking
that this was her period. Then a few days later her real period came – what
should she do: should she make up the prayers of the days she missed or
what?

He replied:  it is better
for her to make up the prayers that she missed during the first days, but if
she does not do that there is no sin on her, because the Prophet (peace
and blessings of Allaah be upon him) did not command the woman who was
suffering from severe istihaadah (non-menstrual vaginal bleeding) and had
stopped praying because of that, to do so. The Prophet (peace and
blessings of Allaah be upon him) told her to regard six or seven days as her
period and to pray during the rest of the month; he did not tell her to
repeat the prayers she had missed, even though making up the prayers she had
missed would have been good, because she may have been negligent in not asking before, but even though she did not repeat them there was no sin on her. 

And Allaah knows best.