Her period stopped for six months then it came back and she has been bleeding for more than three weeks
Question

                I am suffering from irregular periods. Sometimes I have no period for five or six months. The point here is that I got my period three weeks ago, and up till now it has not stopped. Please note that the colour and smell of the blood is that of menstrual blood. What should I do? Should I pray or not? If I have to pray, should I make up any of the prayers I missed?.
Praise be to Allah.
    The basic principle is that the blood that comes from the 
    woman is menstrual blood, so long as it does not last the entire month or 
    only stops for one or two days (in the month), in which case it is 
    istihaadah (irregular, non-menstrual bleeding). This is the correct 
    scholarly view, so there is no maximum limit for the menses. 

    The menses may increase or decrease, and it may come earlier 
    or later, or it may stop for several months. 

    Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
    Some women may remain without menses for four months, then get the menses 
    for an entire month, as if – and Allah knows best – it is withheld and then 
    all comes at once. And some women may menstruate for three or four or five 
    or ten days every month.

    End quote from ash-Sharh al-Mumti‘, 1/402 

    For more information please see question no.
    131541 

    It is well known that the woman who is menstruating should 
    not pray or fast. As for the woman who is suffering istihaadah, she should 
    pray and fast, and her husband may have intercourse with her. 

    Based on that: 

    What is happening to you now is menses, so long as it does 
    not last a complete month or one or two days less than that, in which case 
    it is istihaadah; and take this into account for the following month. 

    See also question no.
    68818 on what to do in the 
    case of istihaadah. 

    And Allah knows best.