Ridiculous!

I currently have a loan with FNB, and last month I paid back almost 80% of the loan and asked them to recalculate the repayments and interest. It seems the staff at FNB don't know how to do a simple recalculation of a loan, now my interest plus fees and CPP adds up to more than what my instalment is. How does this make sense???? This means that I will be paying an instalment every month but nothing will go to the capital portion of my loan?! Yesterday, they deducted 2 instalments and when I queried it they said the 1 instalment is for arrears. I paid almost 80% of by loan back, how can I be in arrears???!!!!! I need a COMPETENT person to call me back today still and explain what underhanded nonsense is going on here!