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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Whoever memorizes the Qur'an or a part of it has to commit himself to it so that he/she will not forget it. The Prophet said: ''Be committed to memorizing the Qur'an I swears by The One in Whose Hands my soul is! The Qur'an slips away more than a camel does from its hobble.'' [Al-Bukhari and Muslim]
Forgetting the Qur'an is an act of neglect and disobedience, it implies that one is not interested in reading it repeatedly. Shaykh Al- Islam Ibn Taymiyyah said: 'Forgetting the Qur'an is due to sins'. Moreover, many righteous predecessors considered forgetting the Qur'an as one of the major sins.
Abu Al-'Aaliyah said: 'We used to consider memorizing the Qur'an and neglecting it until one forgets it as one of the major sins.' [Ibn Sa'd]
Ibn Seereen said: 'They (companions and predecessors) used to hate and severely reprimand whoever forgets the Qur'an.' [Abu Daawood]
Finally, we advise the sister asking the question to sincerely repent to Allaah and repeatedly read what she had memorized and had forgotten. She has to set a time for reading and pondering over the Qur'an because if one does not read it, his/her heart becomes hard.
Allaah knows best.