Negative account balance because of card purchases.

I reached out to FNB to find out how my account got into a negative balance through card purchases but my card does not allow purchases if there are no funds in my account. I spoke to Sanele via chat through the FNB App. He was not helpful and said I must call Authorization.

My question is, how does my account go into a negative when my card does not allow card purchases if there are insufficient funds? The card purchases were deducted from my balane when they occurred and were never reversed into my account at any point. The next thing, FNB will charge R120 for honouring card payment. This is the second time this thing is happening.