Fnb refuses to refund my money

Due to the covid lock down, i missed one months instalment on my fnb loan which was in April. In May well before my pay date i paid for April and my May instalment which would of made my account up to date. To my suprise when i got paid in May fnb decided to deduct my 2 months instalment again after paying, mind you i was short paid in May due to the lockdown. My entire salary went for an instalment that was not owing. I visited a branch and still had no joy, called the call centre and was told i cannot be refunded and the loan was settled even though my loan period has not ended. Disgusted with them and will be changing banks. I have been left without a cent and they are the most inconsiderate institution i have come across.