She miscarried the foetus at two months and stopped praying because she thought it was nifaas
Question

I had a miscarriage at approximately seven weeks. After the miscarriage I did not pray for fifteen days, out of ignorance on my part, because I thought that I came under the ruling of nifaas and I was bleeding.
Praise be to Allah.If a woman has a miscarriage when human features, such as the head and limbs had not yet appeared in it, not even the outlines thereof, then the bleeding that occurs is irregular bleeding and it does not mean that she cannot pray or fast. If human features had already appeared in it, then the bleeding is nifaas. The minimum period in which human features may appear is eighty-one days, as is explained in the answer to question no. 37784 and 45564. 
Based on that, you made a mistake when you stopped praying, and you have to make up these days according to the majority of scholars. Some scholars are of the view that prayers do not have to be made up in such cases, because one was unaware that prayer was obligatory at that time. 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: Based on this, if a person did not do the obligatory purification because the text had not reached him, such as if he ate camel meat and did not do wudu, then the text reached him and it became clear to him that wudu was required in this case, or he prayed in a camel pen, then the text reached him and became apparent to him, does he have to make up what he missed? There are two opinions concerning that, both of which were narrated from Ahmad. 
A similar case is a man who touches his penis and prays, then it becomes clear to him that it is obligatory to do wudu after touching one’s penis. 
The correct view in all these cases is that it is not obligatory to repeat them (the prayers), because Allah forgives mistakes and forgetfulness, and because  He says (interpretation of the meaning): 
“And We never punish until We have sent a Messenger (to give warning)” [al-Isra’ 17:15]. 
If the command of the Messenger of Allah (peace and blessings of Allah be upon him) concerning a particular matter did not reach a person, then it has not been proven to him that it is obligatory. Hence the Prophet (peace and blessings of Allah be upon him) did not order ‘Umar and ‘Ammaar, when they became junub and ‘Umar did not pray but ‘Ammaar prayed after rolling in the dust, to make up or repeat their prayers. Similarly, he did not command Abu Dharr to repeat the prayers when he became junub and did not pray for several days. And he did not tell the Sahaabi who ate until he could distinguish the white thread from the black thread to make up his fast. And he did not command the one who had prayed facing Jerusalem before news of the abrogation of that ruling (i.e., facing Jerusalem when praying) reached him to make up the prayers. 
Under this heading also comes the case of the woman who experiences istihaadah (irregular vaginal bleeding), if she does not pray for a while because she believes that prayer is not obligatory for her. There are two scholarly opinions as to whether she is obliged to make up the prayers, one of which is that she is not required to do so, as was narrated from Maalik and others, because in the case of the woman with istihaadah who said to the Prophet (peace and blessings of Allah be upon him): “I experience severe and heavy bleeding that kept me from praying and fasting”, his response was to tell her what she should do in the future, and he did not tell her to make up the missed prayers. End quote from Majmoo’ al-Fatawa (21/101). 
But to be on the safe side, she may make up these prayers, by doing them in batches, according to what she is able to do. So she can pray five prayers for the first day, then five for the second day, and so on, until she has finished them all. 
And Allah knows best.