Sudden huge internet data usage

I have been Xfinity customer for many years, and I have received 100% internet data usage (1,229GB) warning on Sep. 22.  Today morning (Sep. 25) it shows 1,385GB usage, meaning I have been used the internet data on average 52GB/day.  This is absurd and non-sense, because, since I get the 75% warning notification, I've been checking all network usage in my home and there have been nothing unusual, and made sure to use the network at the minimal.  I even had used my AT&T mobile hotspot for the business works.  The usage history is 509GB in June, 255GB in July and 804GB in August.  At this rate, I expect additional $100 charge in September.  How is this happening all of a sudden with no change in my equipment or usage?  I believe there is something wrong with Xfinity data usage report.