What should a girl do who has reached puberty and fasted the entire month of Ramadan without breaking her fast during her menstrual days, and has not made up those days until now?

A menstruating woman's fast is neither valid nor permissible for her to undertake. She must make up the days she was menstruating. If the exact number of days is unknown, she should fast for a number of days that she believes most likely fulfills her obligation. Furthermore, she must pay an expiation for delaying the make-up fast if she was able to do so in past years. This expiation amounts to approximately 750 grams of the staple food of the country for each day of missed fasting, and it should be given to the poor. The expiation may be given before, during, or after the make-up fast.