How to Make Up Missed Prayers If  postpartum bleeding Ends before 40 Days
Question

I am a woman who had two weeks of Nifas (postpartum bleeding). For seven days I did not see any blood, but I did not pray yet because I forgot that, even though I know that a woman should pray when she is pure (not menstruating or bleeding following childbirth). But I did not think of that. I said to myself: I will pray after forty days. Do I have to make up those seven days or not?
Praise be to Allah.Minimum period of Nifas
There is no minimum for Nifas . A woman may become pure from Nifas one or two days after giving birth. When a woman becomes pure from Nifas, she has to do Ghusl and start praying, even if that is before forty days have passed . 
At-Tirmidhi (may Allah have mercy on him) said: 
“The scholars among the companions of the Prophet (peace and blessings of Allah be upon him), the scholars from the generation after them, and those who came after them agreed that the woman who is bleeding after childbirth must refrain from praying for forty days, unless she sees the clear discharge indicating that the Nifas is over before that, in which case she should do Ghusl and pray.”
It says in Fatawa Al-Lajnah Ad-Da’imah (5/484): “If a woman who is bleeding following childbirth sees purity before forty days have passed, she should do Ghusl and pray and fast , and her husband may have intercourse with her.”
How to Make Up Missed Prayers If Nifas Ends before 40 Days
As you did not pray for seven days, even though you knew that when a woman becomes pure she has to pray, then you have to make up the days that you missed . The way in which you should make them up is by praying five prayers (Fajr, Dhuhr, `Asr, Maghrib and `Isha’) for the first day, then five prayers for the second day, and so on, until you have made up the seven days. This should be done immediately and it is not permissible to delay it, because the Prophet (peace and blessings of Allah be upon him) said: “Whoever forgets a prayer, let him offer it as soon as he remembers, for there is no expiation for it other than that. ‘And perform As-Salah (Iqamat As-Salah) for My remembrance’ [Ta-Ha 20:14].” (Narrated by Al-Bukhari (597) and Muslim (684) 
If it is difficult for you to make up these prayers all together at one time, then you can offer some of them, then rest, then pray some more, until you have made them all up, even if that takes several days, so as to avoid hardship and difficulty. 
And Allah knows best.