She passed blood that was brown in colour and then in the form of a black thread
Question

                My period was late and I thought I was pregnant, but 10 days later I passed some blood that was brown in colour, and on another day it was in the form of a black thread. What should I do?.
Praise be to Allah.
    The blood which means that a woman cannot pray or fast or 
    have intercourse is menstrual blood. Allah has given it signs which are well 
    known to women. With regard to colour, it is black or dark red; with regard 
    to smell, it has a unpleasant odour, and it is also thick. It is blood that 
    comes from the uterus so it cannot be one or two drops and it cannot be like 
    a thread. 

    You are saying that you passed blood that was brown in 
    colour; if you mean that it was brownish, if it comes before the period it 
    is not regarded as menses. If you meant that the colour of the blood was 
    light, then if this was a flow of blood and not just a trace or a few drops, 
    then the basic principle is that it is menses. 

    As for the black thread, it is not regarded as menses. 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was 
    asked: If a woman passes a few drops of blood during the day in Ramadan, and 
    this continues with her throughout the month of Ramadan and she is fasting, 
    is her fast valid? 

    He replied: Yes, her fast is valid. These drops of blood do 
    not matter because they are from a vein. It was narrated from ‘Ali ibn Abi 
    Taalib (may Allah be pleased with him) that he said: These drops which are 
    like a nosebleed are not menses. This is what was narrated from him, may 
    Allah be pleased with him. End quote from 60 Su’aalan fi’l-Hayd, p. 
    6 

    And Allah knows best.