Fnb credit card balance changes overnight without you doing any transactions

My credit card is being **********ly deducted with money and theres no trace of where its going. I transferred money into my credit card and made the balance to be R0.... the next day after no transactions made.. my balance went to negative... this now happend more than 3 or 4 times... and this is only the times I kept my eye on it... how many times before have I been ****** like this I dont know... please be aware of this guys. Its ****************. And when you query this then they just say all looks fine... but its not!!! They do this in a very sneaky way.