Nedbank has ****med me R3 300

I have a Nedbank current account. My available balance - R3 300 and when I go to the branch they tell me that my account is as it should be because I am using an easy account meaning that I sometimes use money which I do not have, I do not understand this as I work on a budget and there was no in any day have I found myself with extra money in my account and end up using it. I have all the notifications that help me track my money. Now it means that from my salary last month Nedbank had the right to my R3 300 since they say I have overspent it. I don't even know where did I even use that R3 300. Nedbank is now becoming a ****.