Teacher, Hr Told Me I Owe 11k

I am an English teacher at a private school in Brazil, with a formal contract. I used to teach 16 hours per week, but around March, I asked to reduce my workload to 10 hours per week. Today (in December), my coordinator told me that there was a problem with my payroll and that, all this time, my salary was still being calculated as if I were teaching 16 hours per week.

I never noticed this because every month I worked a lot of extra hours (sometimes over 40 hours per month, covering classes, administering exams, etc.), and during the two years I’ve been at this school, my salary has never been exactly the same from one month to the next.

Now she told me that I received around R$11,000 more than I should have, and that HR will contact me to arrange a payment plan to return the money. I’m desperate, I never acted in bad faith, and I really like this school—I wanted to keep working here. I'm crying as I write this. I don’t know how this kind of mistake happens, and unfortunately, R$11,000 is a very significant amount for me.