WOW!!! That's a LOT of data.
Do you have a satellite TV receiver connected to the service? If you have Windows 10, do you have the option "Updates From More Than One Place" turned off? Your router is secure, right?
It might be a good idea to install Glasswire on your Windows based computers to monitor the data usage on them. It can tell you what, specifically, uses the data on the computer it's installed on. It's here... https://www.glasswire.com/. It's free. It can't tell you anything about what's already happened, but it can help if there are future data usage problems on the computer.
Unfortunately, the Reps won't be back until Monday to give you any official input. There are others on here that are VERY knowledgeable, though. Hopefully they can help. That's certainly a ton of data to go through.