Sudden Massive Increase in Internet Data

Xfinity customer for many years. Typically use 400-600 GB of data each month under 1.2 TB monthly plan. Suddenly, December shot up above 1.2 GB and over the course of just a few days reached 4.5 TB. Nothing has changed on our end. No new connected equipment. No change in the way we use internet. Xfinity response was to "forgive" us going above plan for one month and see what happens next month. Now that we have reached 4.58 TB of data use, all internet data use seems to have stopped as we have showed zero increase today.  What gives????? Also, every Xfinity person I interacted with by chat and on phone has offered to increase us to unlimited data plan which we do not need and do not want. We need an explanation of why Xfinity shows massive data increase in our home use with a long history of never using that much data.