Hello,
To find out what exactly is using the bandwidth, you would need to refer to your logs. It's possible you have someone or something that is leeching your bandwidth, such as another website linking to files or images. We have also seen people run bandwidth wasting scripts against their competitors or enemies, so you may need to look for large amounts of requests from the same IP address.
In cPanel, we suggest using "HotLink Protection" to keep other websites from linking to your files and "IP Deny Manager" to block any IP's you find that are continuously hitting your website and wasting your bandwidth.
Thank you,