Unexplainable interest charges

Good day

I have a credit card with FNB my is ********** 683082. My credit card is up to date in terms of payment and I hardly ever used it. Last month may I used it once , I used it once at a stores to pay for R 150 worth of goods. But I was charged R 750 for interest and when I asked FNB premier banking they told me that’s the fee you pay if for example you withdraw or transfer money from FNB to another account, and I did none of that. i need help so that someone from FNB can tell me why I was charged R750 for interest,. I feel ****** because what they did is theft you took money that doesn’t belong to you just because you have my bank account.