She wore niqaab during Hajj; is there any sin on her?
Question

A couple of years back i went for the holy pilgrimage of hajj . at the time i performed the hajj wearing a nikab , it has come to my attention that its not permissable for women to cover their faces during the pilgrimage . when i did wear the nikab , i was informed by good sources that it was permissable .
Praise be to Allah.1 – Covering the face is something good for women. May Allah bestow abundant mercy on the women who cover their faces with niqaab. Perhaps your question is based on keenness for your religion – may Allah increase your keenness – but it is better to heed the commands of the religion, which tells women to remove the niqaab from their faces during prayer and Hajj, but especially during prayer, when it is not permissible to cover the face at all except if there are non-mahram men present. During Hajj it is permissible for women to lower the veil over their faces provided that it does not cling to their faces; this is what is called the burqa’. It is not permissible for the woman in ihram to wear it, but it is permissible for her to lower over her face a cover which comes down from her head over her face. If the burqa’ is not available, and there are men present who should not see the woman’s face, then it is permissible for her to wear the niqaab. 
It was narrated that ‘Abd-Allah ibn ‘Umar (may Allah be pleased with him) said: “A man stood up and said, ‘O Messenger of Allah, what kind of clothes do you command us to wear during ihram?’ The Prophet (peace and blessings of Allah be upon him) said: ‘Do not wear a shirt or pants or a turban or a burnous. If one of you does not have any sandals then let him wear the khuffayn (leather slippers) and cut them so that they come below the ankle. Do not wear anything that has been dyed with saffron or turmeric. Women in ihram should not wear niqaab or gloves.”
(Narrated by al-Bukhari, 1468; Muslim, 1177). 
2 – With regard to lowering a cover over the face other than the niqaab, or for fear of being seen by non-mahram men, it has been reported in saheeh reports that some of the Sahaabiyyaat (women of the Sahaabah) did that: 
It was narrated that ‘Aa’ishah said: “We used to go out with the Messenger of Allah (peace and blessings of Allah be upon him) when we were in ihram. If we met the riders we would lower our garments over our faces.
(Narrated by Abu Dawood, 1833; Ibn Maajah, 2935) 
Shaykh Ibn ‘Uthaymeen said: 
It is not narrated that the Prophet (peace and blessings of Allah be upon him) forbade the woman in ihram to cover her face; rather he forbade the niqaab only because it is worn on the face. He differentiated between the niqaab and covering the face. Based on this, if a woman in ihram covers her face, we say there is nothing wrong with that, but it is better for her to uncover it so long as there are no non-mahram men around her, in which case she should cover her face from them. Al-Sharh al-Mumti’, 7/153 
3 – With regard to the past and the Hajj that you did in the manner you describe, there is no sin on you because you were unaware. If a person does something that is not allowed during ihram because he was unaware or he forgot, there is no sin on him, and he does not have to offer any fidyah (ransom). 
It was narrated from Ya’la ibn Umayyah (may Allah be pleased with him) that a man came to the Prophet (peace and blessings of Allah be upon him) when he was in al-Ji’raanah and entered ihram for ‘Umrah. The man had dyed his beard and hair with saffron, and he was wearing a jubbah (cloak). He said, “Take off your jubbah and wash off your saffron, and whatever you do in your Hajj, do in your ‘Umrah.”
(Narrated by al-Bukhari, 1697; Muslim, 1180). 
Shaykh Ibn ‘Uthaymeen said: 
Similar to forgetting are cases where one is unaware or is forced to do something, i.e., if a person forgets and puts on ordinary clothes when he is in ihram, there is no sin on him, but when he remembers, he has to take them off and put on the izaar and rida’ (ihram garments). The same applies to perfume; if he puts on perfume, forgetting that he is in ihram, there is no sin on him, but when he remembers, he must hasten to wash it off. 
Al-Sharh al-Mumti’, 7/222
 And Allah knows best.