i have a hunch it is coming from your blog. If you host a lot of pictures on your blog, with some pictures ranging from 4MB to 7MB
I'm not too sure how your blog/website layout is, but each time each user access those pictures it will count towards your traffic. For example, you have 50 people accessing your site and they viewed a 4MB and a 7MB picture. A rough estimate is already 50*11MB = 550MB for only those 2 pictures.
According to the example that is already more than half of your monthly traffic limit if it was 1GB. Not including other pictures and content that needs to be loaded.
Of course this is just my hunch, you can try limiting your file size to 1MB maxiumum to see if there is a difference next month.