ANDPIE6, going to Settings > Cellular and scrolling down should give a good indication how the data is being used. Also it's a good idea each month when the new data period begins, is to" Reset Statistics" at the bottom of the page to get a good accuracy. Have you looked into these settings?

Has Personal Hotspot been turned on? If that gets connected up to a computer it can go through gig's of data very quickly, especially when the computer starts pulling updates.