When menstrual bleeding is ongoing in one who has just started her periods

Firstly: 

If a woman’s bleeding continues for most of
the month, then she is mustahaadah (suffering from istihaadah or
non-menstrual bleeding). If that happens the first time she menstruates,
then she has no previous cycle to refer to. Rather, in the following month
she should distinguish between menstrual blood and the blood of istihaadah
by noting the well-known signs, namely colour, odour, heaviness or lightness
of flow, clotting, and the presence or absence of pain (cramps). Menstrual
blood is black or dark, has an unpleasant odour, is usually accompanied by
pain, and is heavier than the blood of istihaadah. Menstrual blood does not
clot after it comes out, unlike the blood of istihaadah. If her blood may be
identified in this manner, then whatever matches the characteristics of
menstrual blood is menses, and at that time she should refrain from praying
and fasting, and she should do ghusl when it stops; blood of the other type
is istihaadah. 

If her bleeding is all the same, or it is
inconsistent, and she cannot distinguish between menses and istihaadah, then
she should follow the cycle of most women, i.e., she should assume that her
menses lasts for six or seven days of each month, starting from the first
time she saw the blood; any bleeding other than that is istihaadah. If she
does not know when the bleeding started, she may start counting from the
first day of the (hijri) month. 

Among the reports that speak of
distinguishing between types of blood is that which was narrated by
an-Nasaa’i (215) and Abu Dawood (304) from Faatimah bint Abi Hubaysh, which
says that she was suffering from istihaadah and the Messenger of Allah
(blessings and peace of Allah be upon him) said to her:
“When it is menstrual blood it
is dark and recognizable, so when it  is like that, then stop praying, and
when it is otherwise, do wudoo’ and pray, for that is from a vein.”

Classed as saheeh by al-Albaani in Saheeh
an-Nasaa’i. 

Another report about the woman experiencing
istihaadah refraining from prayer and fasting for six or seven days – if she
did not have a regular cycle before and cannot distinguish between types of
blood – is that which was narrated by at-Tirmidhi (128) and Abu Dawood (287)
from Hamnah bint Jahsh (may Allah be pleased with her) who said: O Messenger
of Allah, I suffer from istihaadah, bleeding heavily. What do you instruct
me to do? For it is keeping me from fasting and praying. He said: “That is
only a kick from the shaytaan, so count your menses as six or seven days,
which is something known to Allah, then wash yourself and when you see that
you have become pure and it is over, then pray for twenty-four or
twenty-three days, and fast and pray. That will suffice you, so do what
other women do with regard to their menses and purification.” 

At-Tirmidhi said: I asked Muhammad [i.e.,
Imam al-Bukhaari] about this hadeeth and he said: It is a hasan saheeh
hadeeth. Ahmad ibn Hanbal said likewise: It is a hasan saheeh hadeeth. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy
on him) said: To sum up, a woman who experiences istihaadah the first time
she has a period should proceed by distinguishing between types of bleeding.
If she cannot do that, then she should base her actions on the usual cycle
of women: so she should refrain from praying and so on for six or seven days
starting from the time when she first saw blood. If she has forgotten when
she saw it, then she should start counting from the first day of each lunar
month. We have stated previously that the most correct opinion is that she
should follow the cycle of her female relatives.

End quote from ash-Sharh al-Mumti‘
(1/490). 

He (may Allah have mercy on him) also said:
The correct view concerning the one who has just started her periods is that
her bleeding is menstrual bleeding so long as it does not last for most of
the month. When the bleeding begins, the one who has just started her
periods should refrain from praying and so on until she becomes pure (i.e.,
the bleeding ceases) or fifteen days have passed. The evidence for that is
the verse in which Allah, may He be exalted, says (interpretation of the
meaning): “They
ask you concerning menstruation. Say: that is an Adha (a harmful thing)”
[al-Baqarah 2:222]. So when this
bleeding – which is a harmful thing – begins, that is menses, whether the
amount is small or great. So how can it be said: refrain from fasting and
praying for one day and one night, then do ghusl and pray, then do ghusl
again when it stops, and make up the fasts?

What that means is that we are obliging her
to do the acts of worship twice and to do ghusl twice; Islam does not
prescribe such a ruling. Acts of worship are to be done once, not more than
that. 

If the bleeding of the one who has just
started her periods continues most of the time, then in that case she is
experiencing istihaadah and should distinguish between types of blood. If
she is not able to distinguish between types of blood, then she should
follow the most common menstrual cycle, or the menstrual cycle of her female
relatives. This is the correct view.

End quote from ash-Sharh al-Mumti‘
(1/49). 

Secondly: 

She has to make up the Ramadan that she did
not fast, because the woman who menstruates must make up the fasts, and the
woman who is suffering istihaadah is obliged to fast, and it is not
permissible for her not to fast. Therefore her days are either menses or
istihaadah, and she is obliged to make up the fast in both cases. 

With regard to prayer, then to be on the
safe side she should make up the days when it is determined that she was not
menstruating. If she was able to distinguish between types of blood, then
she must make up the prayers of the days on which the blood was red and was
not the (darker) blood of menses. 

If she was not able to distinguish between
types of blood, then she should make up anything more than fifteen days,
because that is the maximum length of menses according to the majority of
scholars. 

This is what she should do in order to be on
the safe side, even though she is not obliged to make up the prayers because
she refrained from praying on the grounds that she thought that she was
menstruating. 

Please see the answer to question no.
146190. 

And Allah knows best.