Please explain how you charge an account

Every month I pay my account as stipulated. I then go back a few days later or when I need to to go buy things I need. I always ensure not to spend more than the balance in case of an emergency in the middle of the month. Now every time mid month whenever I want to buy something my balance is always negative. this is very upsetting as I now have to use other monies set aside to purchase those goods. And I get charged for that amount...every damn month. Why should I be paying for something I do not use?