Overdue account??? Interest???

I took out a personal loan from capfin. The repayment was over six months. Yesterday I made my last and six repayment towards the loan. Now they call me and tell me that R120 is overdue because of interest. My husband made his last payment to capfin last month and does not have any amount overdue. The consultant who phone me insist that I have this amount overdue because of interest. When I asked her why because my husband didn't have any she tells me that is just the way it is.<br> <br> Can Capfin please help me understand why I have to pay this interest money if at the beginning of the contract they gave me the installment amount with interest already calculated in this amount?