FNB BAD ADMINISTRATION

I had a Discovery credit card that I paid up in April 2018. Then about two months later I started getting calls from FNB (the credit provider for Discovery) that I was just over R800 in arrears. Every few days I would get smses, / calls and so it went on until Sept 2018, and every time the amount went up substantially. In Sept, I received a statement showing a credit amount of R1750. I was happy. It was short-lived. In summary, I have been "handed over" to three difference collection agencies, the amount has now increased to over R1700. They would call, send smses and the last one emailed me to which I rep**** that I owe nothing and sent them the last statement I received in Sept. I also told them that I am prepared to go to court. They are quiet for about two/three weeks then start over again. I am beside myself. My question to FNB is this: if the last statement shows a credit balance why have I been handed over? And why to three different agencies. There is something seriously wrong with their administration. I was contemplating going over to FNB a while ago, but now after this fiasco, NEVER IN A MILLION YEARS!