Incompetence

I paid off my loan on the 21st of February, to my surprise fnb deducted monthly installment from my cheque account on the 24th, plus charged interest on the 25th, and some auto credit amount I don't understand. I sent a complaint to them but have not responded. I also noticed that there is a next date of installment for the 25th March, how the do they arrange for the next date of payment on a zero balance, where is my money going? I want my money back asap, I've got other debit orders that needs to be paid. They took money from me but when I check my loan balance it's still R0, where did they take money to?