Inconsistencies in bank statements

I have been a Nedbank account holder for years now. I woke up this morning and noticed that my one account had been withdrawn by R2000 on a debit account where there is no credit limit. This transaction was apparently due to my purchases from TechExpress over a month ago which had been transacted. Upon further investigation of my statements on the Nedbank site I had noticed that the sms notifications I was getting was not in line with my actual bank statement and there were many large discrepancies. I am never allowed a credit limit on any of my account and now suddenly I am either being double charged or something else fishy is happening in my account. I tried calling the Nedbank line but they are closed on a Saturday. I need some answers because now I am apparently in a good sum of credit. If the account were up to date and the notifications were accurate then I would never have allowed this. I was even swiping my card and checked on the Nedbank app and saw I had a positive balance.