Installment reversed, now I am owing!!

My installment went off my account on December 24 as per arrangement with African Bank. I have always paid all my accounts with them via debit order, never missing an installment to the end of my loans. Suddenly on the 4th of January my December installment was reversed back into my bank account. Then came this sms: Your AB payment has been disputed and reversed. This is not in terms of your agreement. Please call ...today to avoid legal action being take.<br> I want to know what is this? I will not waste my airtime on this and you better not charge me interest on this installment. You reversed it yourselves, the same amount you debit me every month, what has changed? Please contact me and tell me you have fixed this. This is my 3rd loan with you, I have NEVER missed an installment ever. I always honour my installments to the very end.<br> This is most annoying.