Bad customer service

In mid June I bought tickets for my parents online, money was taken out of my bank was left with R2070, the following day I sent my money R2000 after a day I bought R10 airtime and R50 electricity. To my surprise after a week I'm getting a call from Nedbank employees telling me that I over spent on my card, that I bought flight tickets with money I didn't have. Please explain to me how do I spend money I don't have when I don't have an overdraft or credit card? And Why I my transaction different from the bank. I'm tired of being harassed by Nedbank employees. I don't remember asking the bank to loan me any money. If they had system errors why are they harassing me for the mistake on their side. I want that account closed and sorted as this is going to affect my credit score. I wouldn't have sent my mom money if there was no money in my account.