Data Spike in Usage completely over normal use

I got a notice that my data usage hit the limit this month, with it spiking nearly 3x usage over the last couple of weeks. My household has changed nothing in our browsing behavior. As well when I look through my individual device data logs, the numbers don't add up. For example, Xfinity tracked 400GB of usage on May 16th, with 86GB of that from my phone on that ONE DAY. My phone only shows 60GB of Wifi usage over the PAST 30 DAYS. I assume that what is coming through the router is what Xfinity is using, but these numbers cant be right.