Wes's laptop died after he dropped it last week and he got a Windows Surface tablet. He got it with LTE and found out within a few days that he had used all of his data. Leo says there's got to be an app that is phoning home and eating up data. It could be that his tablet tried to download all of his mail at once. When he first set it up, it likely downloaded updates as well. Windows Threshold is a 3GB update all by itself. And then toss in other apps, and it's likely that's what happened. Leo also advises calling Verizon and request information on what it was connected to and what traffic was flowing through it.
The chatroom says to check Task Manager --> App History --> Network. That'll tell the tale on what apps are using how much.