Crazy Spike in Internet Data Usage

I am experiencing a similar issue. I am hoping rebooting our modem will solve the problem and we don't have to deal with Xfinity trying to upsell us a new plan or new modem or blame it on us when nothing in our viewing/Internet habits has changed

A lot of things cause data usage, and every time you connect anything to the internet, you are doing something different.  Web sites auto-loading audio/video -- some malicious, some playing video you can't even see in the background, cross site scripting attacks to load content, program/OS updates, phones/backup programs getting stuck slamming backup data to cloud services.

Assuming you don't have hijacked devices (I see lots of hijacked granny comcast machines hitting my firewall):

Streaming services are not equal.  There's a lot of 4K/8K content out there.  That can cost you as much as 22GB/hour.  (you get 40GB/day average on a 1.2TB data cap).  A cable box pulling the cable lineup doesn't eat your data use, but if you watch Netflix/Hulu or another streaming app on the cable box, you will burn data.  A lot of it.  Cable boxes can really pull down data well.

Got Xfinity equipment and close neighbors?  If you don't have an MoCA PoE (point of entry filter) installed, your neighbor's devices could be connecting to YOUR network.  There is no security on Xfinity crippled gateways for the Coax.  Anything you don't recognize if you look for connected devices?

Did you do any port forwarding (opening up ports in your firewall) for gaming devices or are you running a publicly accessible service on anything like trying to do a remote desktop/allow remote access?  Got any uPnP gear like cameras (Nest...etc), or other IOT gear like smart outlets blowing holes in your firewall and establishing remote connections?  All can be problematic.

Best practice?  Run all devices including WiFi traffic through a firewall/firewall appliance, and run your own WiFi network NOT on a gateway.  Firewalls like   Pfsense/OpenSense/Firewalla or even just a Linux box with two network ports.  Some mesh networks have decent logging....etc.  Then you can see and log what is using data.  Even throttle and control data with traffic shaping on some of them.  You can't with an Xfinity rental gateway.

The assumption -- I'm not doing anything different is incorrect.  It changes all the time.

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The first thing I would suggest you do is to try reaching out to our Customer Security Assurance Team. They are experts at digging deep into these things. You can reach them at this link, https://internetsecurity.xfinity.com/help/report-abuse. I promise you that they are the best people to help you with this! 

REALLY!  This is crazy..i look at my data 3-6 x a day to see how much its changed & i went from 400-800  l@ month to, starting in feb i was at 900 march almost 1200 & now i have 3 days left in april & i am at 988. [Edited: "Language"] I do nothing extra as far as gaming i watch the same 5 channels i been watch wen i got this [Edited: "Language"]  Hulu,Tubi, espn, disney plus & xfinty stream only wen these basketball playoffs are on. Its [Edited: "Language"] they been tryin to sell me the unlimited data deal.11.99 @ month but im done with them as soon as this contract is up i been with them 4 yrs only caus they tricked.me into another 2 yr deal & this time i got my month mixed up wen it was up & started another 12 months i believe 4 months ago ima look wen i check my data again in an hour.  . [Edited: "Language"] i slept until 4pm yesterday with no t.v. on i was sick from pollen & my data was havin a party im still tryin to figure out wut used the data from tuesday nite at 2am until Wednesday at 4pm but somethin did

Yep having the same issue I literally just blew through 1229 gigs 17 days and I still have 2 weeks to go. Have not heard anything back from xfinity. I think it might be time for a change in companies