Assalaamualaikum
May Allah fulfill your wishes.. Jazakallahu khair for your effors... Its very helpful..My question is I had a 37 day nifas.... Then 8 days purity....then i saw Few drops of red blood along with pain... Is this mensus or istihada?
Because i had only 8 days of purity in between and for mensus it has to be 15 days right?
Jazakumullohu khair

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. 

If the Nifaas (post-partum bleeding) stops before forty days and then the woman sees blood again within the forty day period, it is considered Nifaas blood (lochia) until the forty days end. Any bleeding beyond the forty day period is considered Istihaadhah (vaginal bleeding outside the menstrual period) unless it coincides with her regular menstrual period; in this case it is to be considered menstruation.

Shaykh Ibn Baaz said, "If the woman becomes ritually pure before forty days and then the bleeding resumes within forty days, then the correct view is that she should regard it as Nifaas within the forty-day period, but her fasting, prayers and Hajj at the time when she was pure are still valid and she does not have to repeat any of them, so long as they were performed when she was ritually pure, until she attains purity (again) or the forty-day period ends." [End of quote]

For more benefit, please refer to Fatwa 91527.

If the bleeding exceeds the forty days, it is regarded as Istihaadhah. In this case, the woman should perform Ghusl  after the forty day period comes to an end and she should use a liner of cotton or pads, and perform Wudhoo’ before each prayer. This is, of course, unless the bleeding coincides with her regular menstrual period; if so, it is to be considered menstruation. Please refer to Fatwa 84539.

The Shaafi‘is and others held that the maximum length of Nifaas is sixty days. This scholarly view carries considerable weight, and the versed scholar Ibn ‘Uthaymeen also preferred it. So, there is no harm in you following this opinion, based on which, there is no problem with the blood beyond the forty-day period because it is considered Nifaas within the sixty-day period according to this opinion.

However, based on the first opinion, which is the one we prefer at Islamweb, the bleeding you have experienced within the forty-day period is Nifaas and beyond that it is Istihaadhah unless it coincides with your regular menstrual period.

Allaah Knows best.