FNB CHARGING INTEREST ON SETTLED ACCOUNTS

I have settled my loan with FNB last week; I sent them the proof of payment on the same day and asked for a paid up letter. The amount was credited to the loan account, but now they are adding interest to a non-exisiting balance. I have been on the phone for the last 40 minutes and I am being transferred from one person to the next. All I want is for the loan account to be closed and someone to send me a paid up letter. Also when I am calling about a loan account; why do you need my bank card number and pin???? No one can answer me this. I used to be the person who gladly recommend FNB, BUT now I see what everyone has been warning me about.