disappointed & angry

Just before lock down i was helping my mother with her accounts payments because she couldn't go to the shops anymore due to lock-down and i helped her to make payments online.she was advised of the balance and she made payments as arranged and before she made a last payment to them they confirmed the balance and now all of a sudden the account went up to +- R400 without purchasing anything. and none of them advise her that the account will accumulate without any purchases

how the hell do they do that to people (mind me a pensioner) who have always been with them for a long time and was a reliable payer. this is a ridiculous shop who eat from the poor/elderly