Huge increase in data usage according to Cox

I've been a Cox customer since before they were Cox, it was Dimension Cable when I first signed with them. For years, I never exceeded 300 gigs of data a month even when the grandkids were here watching Netflix and using their iPads. Two billing cycles ago I received an e-mail saying I'd reached 85% of my 1T of data. The only devices in the house are 2 desktop computers and 2 iPhones. The following month, I exceeded 1T of data according to Cox. I thought maybe someone was stealing my wifi so I replaced the modem and router and changed my passwords 2 weeks ago. I've spent many, many hours trying to figure out why, after changing out everything, I'm still using 50 gigs of data a day unless I unplug the ethernet cord when I'm not using it. I turned off the wifi on our phones and disconnected the other desktop but I'm still being told that I'm using massive amounts of data using one desktop computer and not streaming anything! The Cox technician just left my house after installing a filter on my modem so I'm not going to unplug tonight and see what happens. I'm afraid I will have to cancel my account with Cox because I'm not going to pay for unlimited data when I KNOW I'm not using the data they say I'm using.