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, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Shaykh Ibn ‘Uthaymeen said: "If this woman was sick and could not make up for the days that she did not fast in the month of Ramadan, she should fast these days as soon as she is able to do so and she would be excused for this even if the following Ramadan has come. However, if she has no excuse and she is simply negligent, it would be impermissible for her to delay making up for the days that she has not fasted. ‘Aa’ishah said, “I could not fast the days that I did not fast during the month of Ramadan except in Sha‘baan.” Thus, this woman should know that if she does not have an excuse, she would bear a sin and should repent to Allaah and make up for the days that she did not fast. If she had an excuse, she will not bear a sin even if she delayed making up for her missed fasting for one or two years."
Allaah Knows best.