Money being taken for no reason

I have a credit card with the bank. They gave me R6000.00 to spend and they take R750.00 per month and put it back on the card, they then take an amount of +- R200.00. This month I wanted to withdraw R400, when I checked the balance it was R150.00. I went to the bank and asked for a statement to show where the money is. No one could tell me and the statement doesn't show the money. I then asked how much do I owe in total, they could not answer me, it is very disappointing to get such bad service as a loyal client. I was willing to pay the full amount, but they could not give me an answer and now some of my money is just gone, with no reason. They told me I should just pay every month R750 without any guarantee that the money will be there or for how long I would be paying. Please can you help in this situation?