You need to go into your cpanel and check out AWSTATS or Webalizer to see where that bandwidth is going. Both of these utilities break down your traffic into stats that show who is using what... You will never figure out what's going on by looking at your configuration, see what the numbers are and who is sucking it all up on you. You may be having some axxhole hotlinking to your site or like I just discovered, a hacker may be serving porn from you site without your knowledge. Took my site down in only one day... he "planted" his porn distribution in my /xml folder. I had no idea until my site went down for exceeding bandwidth.
Check it out, you might be okay but find out where that stuff is going!
For sure, don't guess...