Penalised for early payments??!!

I am utterly disappointed in the way Truworths handled my account query. I opened an account with them in May 2016. Since opening and purchasing from them, I have religiously paid my account on time every single month without fail. This month I received a statement with no description of payments other than an amount billed as interest ( all purchases I have ever made was on the 6 month interest free option) so this puzzled me. Then I noticed that my instalment was double the amount I should be billed. A day after I received my statement via email, I receive a call from an attorneys office claiming my account is in arrears and I MUST pay in R1000 to get my account up to date. I explained to her that I have paid my instalments every month and have the receipts to prove it, so she advised me to go in store and have it queried. I did so, and made this month's instalment payment and asked the cashier to look up my payment history and found that I have paid every month and then she called head office. They told her that because last month I paid an instalment before the 16th but after the 6th, its seen as a late payment???!!! I can't understand this? So I am now charged interest for paying my account before the time the statements are sent? This doesn't make sense to me and I left the store more puzzled than ever. I then received more sms's and another call from the attorneys demanding payment for arrears, by then I was annoyed by their persistence and decided to call the head office, Cape Town myself. I called them on Saturday the 19th May at 11:05am and spoke to Nomwabo. She looked up my account and told me the same thing, that my account was paid late in April and that it is in arrears. I then explained to her in detail that I have paid the instalment due every month and have the proof thereof, yet she insisted that last month's payment was made late, therefore my account is now in arrears and said next time I must let the cashier know I am making an early payment in order not to be charged interest?? So what happened to March's payment that was due before the 6th April but paid on the 20th March?? And what happened to April's payment that is due the 6th May but was paid on the 9th April?? Why am I being charged R23 interest?? According to me, my account has been paid every month and the system should pick that up irrespective of which day the payment was made!!! She insisted that I was in the wrong and that I must pay the interest and she could not rectify or remove it. I then asked Nomwabo for her name, which she reluctantly gave me and her surname which she refused to give me and asked her for a reference number for the call which she said there wasn't one. I would like this matter to be addressed urgently and I refuse to be charged extra for nothing. I am also disgusted that I was handed over to attorneys for "one late payment" which wasn't even a late payment and after two years of paying on time....