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

There is a girl who is sixteen years old. She reached puberty on the first day of Ramadan (i.e., got her first period), and the bleeding continued without stopping until the end of the month, so she did not fast for the entire month. What must she do?
Praise be to Allah.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.