FNB RANDOMLY DEDUCTED A PAYMENT A MONTH EARLY

I recently took out a personal loan from FNB in March. According to my online Banking, the first payment was due end of April. Today, FNB deducted an instalment, a month earlier than stated from my account. I am furious - I have other debit orders that go off today but now there is not enough money in my account for that! I did not budget for it to happen end of March already because it was communicated that end of April was the first deduction. I would like this to be fixed IMMEDIATELY. You cannot do this when the first installment was due end of April. FNB, please fix this now!!!