I must add, it is very easy to run up broadband usage and get caught out without realising, INCLUDING when you are not at the computer. I'm not saying this is the case here, but think carefully about your past few days/weeks usage.
For example, I recently reinstalled Windows, and then re-downloaded some games on Steam. Never gave it a second thought until I got the email warning me that I was close to my limit. And Steam could be continuing to update after you walk away from your pc.
When I checked, I had actually downloaded about 15GB of software from Steam, (plus my wife had watched some golf online for best part of a day). Before you know it, that's a lot of GB used up.
So it's worth checking back on your usage as well as looking for the unauthorised causes.
