Amounts

Every month there is a big amount in my bank but then suddenly without using my card my account is in a minus. When they send me the new card with the account they told me that it doesn't matter how many times I use my account the monthly amount is R100. If I go to any of there branches they told me that It is places what still have to go through, but there isn't anything. All I want to know why must there be almost R1000 in my bank but my account is in the minus? I want it all recovered asap!!!!