Blood of nifaas returning on the fortieth day
Question

                I gave birth and the bleeding lasted for two weeks, during which it sometimes stopped Then during the next two weeks or so I had some yellow spotting and I saw the sign of purity (tuhr) on the 34th day of nifaas, so I did ghusl and prayed Zuhr, and my husband had intercourse with me, then at ‘Asr time I saw some yellowish spots, and two days later I started bleeding again, until the end of the 40 days, when I saw the tuhr so I did ghusl and prayed, but I was still bleeding very slightly. Now since yesterday I have been bleeding continually and I do not know whether I am in a state of purity or still in nifaas? What is the ruling on the days during which I had the yellowish spotting? Do I have to make up the prayers that I did not offer during those days or was it nifaas?.
Praise be to Allah.
    If a woman bleeds because of giving birth, then she is in 
    nifaas, and remains so until she sees the tuhr or completes 40 days. 

    Based on that, what happened to you before you saw the tuhr 
    was all nifaas, even if on some days you saw yellowish discharge, because 
    yellowish and brownish discharge, so long as it is connected to nifaas, is 
    part of it, so you do not have to pray during that period. 

    Shaykh Muhammad al-‘Uthaymeen (may Allah have mercy on him) 
    was asked about a woman who saw the blood of nifaas for two weeks, then it 
    gradually turned into a thick yellowish substance, and continued like that 
    until the end of the 40 days. Does this substance, which came after the 
    blood, come under the ruling on nifaas or not? 

    He replied: This yellowish discharge or mucus, so long as no 
    clear sign of purity appeared in it, comes under the same ruling as the 
    bleeding, and she is not pure until she is rid of it. End quote from 
    Fataawa al-Mar’ah al-Muslimah, p. 304. 

    As for your doing ghusl and praying after you saw the tuhr on 
    the 34th day, it is valid, praise be to Allah, and your husband’s having 
    intercourse with you after that is permissible. As for the yellowish 
    discharges that came after that, they were not part of nifaas, because they 
    came after you became pure (tuhr). Abu Dawood narrated in his Sunan 
    (264) in a report which was classed as saheeh by al-Albaani, that Umm 
    ‘Atiyyah (may Allah be pleased with her) said: We used not to regard the 
    brownish and yellowish discharge after the tuhr (end of the period) as being 
    of any significance. 

    Based on that, prayer during these days was obligatory, so if 
    you did not pray, you have to make up those prayers. 

    As for the resumption of bleeding following that, it is 
    nifaas if it occurred within the 40 days. Al-‘Allaamah Ibn Baaz (may Allah 
    have mercy on him) said: If the woman in nifaas becomes pure within 40 days 
    and fasts for a few days, then the bleeding resumes within 40 days, her fast 
    is valid but she has to stop praying and fasting during the days on which 
    the bleeding resumes, because it is nifaas, until she becomes pure or 
    completes 40 days. End quote from Majmoo’ Fataawa al-Shaykh Ibn Baaz 
    (4/133). 

    So long as you saw the tuhr (sign of purity) at the end of 
    the 40 days, then your nifaas had come to an end and your ghusl and prayer 
    were valid. As for the resumption of bleeding after that, this is menses, so 
    long as it does not go beyond the maximum time for menses. Shaykh Ibn 
    ‘Uthaymeen (may Allah have mercy on him) said: If the bleeding resumes after 
    40 days, then it is menses unless it continues beyond the maximum time, in 
    which case she should stop praying, fasting and so on for the duration of 
    her normal menstrual period only. End quote from Majmoo’ Fataawa 
    al-Shaykh Ibn ‘Uthaymeen (4/289, 290). 

    Thus you will know that the days when you were bleeding after 
    40 days are menses, even if this bleeding was interrupted, provided that it 
    did not last longer than the maximum time for menses, which is 15 days. 

    But if this bleeding was very light, as you say, such as a 
    drop or two, then it is not menses and does not prevent you from praying or 
    having intercourse with your husband, but you have to do wudoo' for each 
    prayer after the time for it begins. 

    And Allah knows best.