Blood of nifaas returning on the fortieth day

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.