Maybe you server is hacked, maybe you have big files being downloaded because they were crawled by google or any other search engine.
Do you have any stats analyzer on your website control panel? The panel provided by your host. If you have WHM/Cpanel you can use Webalizer for example, that lists you traffic data like the most downloaded/accessed files and how much bandwidth they used.
If you need more info just post it here and others or me will try to help you out