Maybe your computer is downloading updates in the background. (There are several programs that have automatic updates). If you have windows 7 or 8.1 maybe it is downloading windows 10 in the background -- did you sign up to reserve windows 10? You should have windows update set to check for updates and choose when to download. With Windows 8.1 and Windows 10 you have to set your connection to metered or updates download automatically. To set metered you must be on a wireless connection. If you have wired you can not set metered.
Hello dlubarsk1, I would be more than happy to review your account details and check your usage categories so we can figure out where exactly your data is being consumed. Please send us an email to firstname.lastname@example.org with your account and contact information. Thank you