Incorrect deduction of balance

I had a personal loan which its balance was R980 due on the 15 May 2018 and i received a letter to confirm same on the 16 May 2018. On the 15 May the last deducted and balance was R1540, why such a lot i dont know. I called twice and unfortunately Tintswalo was sooooo rude so no clear and correct answer came out of it. Can Nedbank refund me the balance of R560 as i dont know why such a lot of money was deducted?