Dought & clear - She saw menstrual blood after breaking thefast, but she is not sure whether it came before or after breaking the fast.

One day during Ramadan, shortly after breaking the fast, I saw
menstrual blood, but I do not know whether my period began before
breaking the fast or after. Do I have to make up the fast of that day
ornot?
Praise be to Allah.
One of the basic principles of fiqh that were mentioned by the
scholars (may Allah have mercy on them) is the principle that every
incident is presumed to have begun at the closest time.
What this principle means is that if something happens, andit may have
happened ata time that is closer or a time that is further, and there
is nothing to suggest which of the two times is most likely, then we
assume that it happened that the closerof the two times, because this
is the time at which we may be certain that it happened,whereas the
other is the subject of some doubt.
One of the applications of this principle is that if a person sees
maniy (semen) on his garment, and he knows that it is the result of a
wet dream, but he does not remember any wet dream, in that case he may
attribute it to the latest time that he slept and repeat any prayer
that he offered after that sleep.
This principle was statedby az-Zarkashi in his book al-Manthoor
fi'l-Qawaa'id and as-Suyooti in al-Ashbaah wa'l-Nadhaa'ir; they
alsodiscussed some minor issues connected to it, which you may read in
either of the two sourcesmentioned for further information.
Based on that, if a woman saw menstrual blood and does not know when
it began, was it before Maghrib orafter, then in this case she should
assume that the bleeding began at the closer of the two times. The
closer of the two times in your case leads to the conclusion that it
began after Maghrib.
It says in al-Mawsoo'ah al-Fiqhiyyah (26/194): Under this heading
comes the idea that was narrated from the fuqaha': if a woman
seesmenstrual blood and does not know when it began, she comes under
the same ruling as a manwho sees maniy (semen) on his garment and
doesnot know when it happened. That is, he has to do ghusl and repeat
the prayers from when he last slept. This is the least complicated and
the clearest of the scholarly opinions. End quote.
Shaykh Muhammad ibn Saalih al-Mukhtaar ash-Shanqeeti (may Allah
preserve him) was asked about a woman who saw some menstrual blood
after Maghrib prayer, and she does not know whether it came before
Maghrib or after – what is the ruling on her prayer andfasting?
He replied: If she saw theblood and thinks it most likely that it came
beforeMaghrib, then there is no doubt that the fast of that day is
invalid and she has to make it up.
But if she thinks it most likely that the blood is fresh and that it
happened after Maghrib,then there is no doubt that her fast is valid
and she has to pray Maghrib when she becomes pure (i.e., her period
ends and she does ghusl); she should make it up and offer that prayer.
But if she is uncertain, then the basic principle according to the
scholars(may Allah have mercy on them) is that it shouldbe attributed
to the closest time. The basic principle is that the fast remains
valid unless there is evidence to prove that it is not valid, and the
basic principle isthat she has fasted a whole day and has done what is
required of her, unless we are certain that there is any evidence to
the contrary.So in this case her fast is deemed to be valid. As for
the blood, it does notaffect that day. On the other hand, if you say
that her fast is valid, she has to make up Maghrib,and if you say that
her fast is not valid, she doesnot have to make up Maghrib, so if her
fast is valid she has to make upMaghrib, because the time of Maghrib
began when she was not menstruating, and her menses began after
that,so she has to make up Maghrib, because what matters is the
beginning of the time (for Maghrib), not the end.
End quote from Sharh Zaad al-Mustaqni' by Shaykh ash-Shanqeeti
To sum up: your fast is valid so long as you are not certain that the
bleeding started before Maghrib.
And Allah knows best.