Dear Maria Andrea
Mitosis and meiosis refers to the cell division types in which chromosomes are shared, that is the hereditary material at the moment of division.
During this proces mistakes could happen in both, ( mitosis and meiosis) affecting all of our chromosomes, in your case has been the " X " chromosome. To complement this information go to the index web and take a look at: CHROMOSOMES.
To understand the concept, " gonadal mosaicism " go to the index web,....clik on QUESTIONS...... and look for question number 7.
Best regards
Maite Solé