Item paid no funds

I tried handling this issue in the secure chat but 3 consults later I still haven’t been assisted. Fnb is known for random transactions that happen in your account that can’t be explained. I decided to enquire on one that happened described as “Item paid no funds” I don’t have overdraft, I always get charged R8.50 for declined transactions but according to them I was able to pay wit insufficient funds and thus charged for it after. It’s so frustrating trying to make sense of this