Sudden Data Overage!

We have had our home WiFi for 3 years now, and have suddenly gone well over the data limit for the past two months. Last month we maxed out the additional allowances, and are headed for the same this month with 4 days left. I am at a total loss, as we have far fewer devices connected than ever before!

Until last month we had the following connected to the home WiFi: A 4K smart TV with attached Fire Stick and PS4 streaming PS VUE, another non-smart TV with a Fire TV, home VPN for full-time work from home, occasional iPad/Kindle, and two smart phones. He also used to play games on the PS4 with friends up to 10 hours a week, including downloads and updates. We never went over the data limit.

Recently, we received an unlimited data plan for our phones, so they were removed over a month ago, and have not been connected to the WiFi since. Since the first overage, I identified the connected devices (our WiFi is not being used by any outside source that I can see, and we still have the complicated AT&T password) and completely removed the Fire Stick from the smart TV, have not connected iPad/Kindle one time, and completely disconnect the Fire TV when not in use. VPN usage has not changed in over a year. That means that, at most, there are two devices connected at a time where there used to be upwards of 5 or more at a time. 

Can someone please explain how we are all of a sudden burning through data with LESS usage???