Bad communication interest must come down

I send a letter on 20/01/20 to change my debit order to 21 of each month. Reason Education Dept change payment date to 20 of each month I send my payslip Today I receive a notification of change Date show debit order 20 and not 21 . My big problem now interested come down to 10%last month my Payment was R5 842.67 and now R5 838.29 was R 3221.87 and now R1 622.61. Interest was R2 551. 80 now jump to R4 146 . 68 How can interest go up if the interest com down Please ABSA Ihave been a client for 36 years how can you make such a mistake