Unexplained charges on my account

on the 10th August I had a charge on my account of R180 and when I made a follow up with my private banker she said it was for a purchase I made whilst there were no funds in my account. I don't honestly understand, because before I buy anything I always check if I have sufficient funds on my account. Today again there is another charge to my account for R45, I honestly don't understand what this is for? I also got charged for printing a statement at an ATM, I thought 3 months bank statements are free. Could you please explain to me as to what the R180 charge was exactly for which/ what purchase I made and also the R45. I always make sure that there is enough money in my account for my debit orders and whatever loans installments that have to go off. Why would I all of a sudden make a purchase on my account knowing very well that there are insufficient funds?