Unknown overdraft from FNB

I’m writing this with disappointment and unfair treatment I received from FNB. On 16 August 2021 I had a bank balance of R3329.00 at the end of business day. That evening I bought R15.00 airtime through my phone. On 17 August 2021 at around 17:00 while reading my messages on the FNB app I saw that after buying the R15.00 airtime, R3329.00 I had a remaining balance of R955.00 meaning that R2359.00 had been taken from my account without any notification on my phone as it usually happens. On 18 August 2021 I went to the FNB branch in Vryburg to seek an explanation for this. I was assisted by one Ma. K. Mogapi. She went into the system but the missing money was not appearing on the system. I was then put on the phone with someone from FNB head office. Initially this person told me that I had an overdraft which is something I dispute. Secondly he told me that I could have mistakenly taken the overdraft on the app but I dispute this too. Thirdly he said that I may have consented to it but that it was never explained to me that it was an overdraft. I wish to state that I do not believe that I have taken the overdraft or use the overdraft. It doesn’t even appear on my accounts. I was only there to find out about my missing money. I strongly suspect that this has been happening all along and that funds have been taken out of my account without notification and my knowledge. Pleas help!