Early debit order before i got paid

This morning i transferred money from my savings account with another bank to my cheque account and was shocked to see that Nedbank deducted the installment for my personal loan early. I cannot believe as my debit order is set for 20th of the month and i haven't even gotten paid as well. How is this possible? Is this not ******* as i am not a bad payer and pay each and every account every month? I want an explanation in writing from Nedbank as soon as possible.