No purchase, how is interest calculated.

My statement way emailed to, on opening, I noted that my balance was R108. 88, this is for the club card and service fee, no transactions, so I did not have any other to pay, so I go to the the Pavillion branch of Edgars to pay for the amount of R108. 88,I got to the cashier, I handed my Edgars card to her and I asked her to please tell me what is the balance, her answer to me is I must pay first I asked why is this so because in the past they would swipe the card and they would tell us the amount to be paid. I agreed and paid per EFT and to my surprise the receipt showed approve payment of R108. 88, but there's a balance of R111. 00 now I queried this amount and the cashier could not give me an answer, another cashier or supervisor was asked to assist me, after explaining and showing her the statement that was emailed to me, she called the account queries, I must have been on the phone for well over half an hour, the person that was handling my query was Nolukholo, after explaining her as well, she kept on asking me to hold and then come back and again no answers to my query, just to go back, this has happened and a couple of times, Then after me being on the phone for all the time, she mentioned that she has escalated my query to the Team Leader, her name is Zenaria, she will call me, to date I have not received a call,. Tuesday 13/10/20 around 14.15pm I received an SMS, Stating that due to a technical fault the interest was not charge to my account and the interest would be double on my next statement, I rep**** to the SMS requesting what interest would that be because I did not purchase anything clothing and my account is a 6 month account. I am not sure how you charge interest if you have not purchased any clothing, I use a credit card and when I pay the full amount before the 55 days no interest is charge, Edgars is involved in a business rescue, for all the years I owned this account, never did I not pay and now are they trying to extra money from me, I am a pensioner and is poor from a company like Edgars, and no one calls back.