Data

There are a number of things that can all of a sudden start using a LOT of data. Someone in these forums several months back had a similar issue, then found he had installed one drive on his phone, and it would sync to one drive and upload from, his phone the entire phone, and use close to 100gb each time. if you did that with several devices, well, adds up quick. 

Also, while streaming habits may not have changed, the resolution of the streaming MAY have, and while standard HD uses, 2.3 to 2.8gb an hour, 4K tv uses 8gb an hour and more. 2 movies in an evening, could go from using 10gb, to using 32 gb, and if you have 2 people each on their own tv, that could easily double from  instead of 20gb,  to using 64 gb and that's just for 2 tvs each watching 2 movies at 4k. it doesn't count what devices like tablets or phones, computers, and more could be using. 

Where are the emails from? Do you have any way of fact checking your bandwidth meter? Maybe a router with it's own meter?

As a first step, I would go to https://www.cox.com/internettools/data-usage.html and check your usage.  Additionally, you can manage your notification preferences from that page.

Almost certainly something akin to what others have described. You’re probably having background updates/syncing using data. You can view detailed data usage by device if you’ve got the apps installed. You can also limit the data they can use. Without a detailed list of the type of devices you use / services you have / etc that is about the most anyone can explain. If you’re able, ask them about changing service to an unlimited data plan. My internet is very reasonable for price and it’s unlimited. Not sure it’s kosher to talk about pricing on these forums but really, just ask they are willing to help if you’re a customer that pays on time every time and courteous to the reps.