Data Usage

In 10 years of using Xfinity internet I don't believe I've ever went over 300GB in a single month and it's usually much lower than than. Last month my data usage jumped up to almost 1200GB so close to going over the 1229GB allowance. I use my own modem/router and it tracks my data usage which says I only used 280GB last month. So far in just 9 days this month Xfinity says I've already exceeded 350GB which is impossible unless I were streaming/gaming no stop all day every day which I'm not doing anything different than I ever have. I've contacted Xfinity several times about this issue and they don't seem to care. Either my router's data usage tracking is wrong or Xfinity's is, considering my data usage history and the astronomical usage I'm experiencing in just 9 days this month I'm going to guess something is wrong on Xfinity's end. I've spoken with the Security Assurance department and wow that was a waste of time. In fact so far everyone I've spoken with at Xfinity has been a huge waste of time. I still have my courtesy month for going over my data usage but I sure wish Xfinity would show a little initiative and help me figure out where all of this is data is going before it gets to that point but that might be too much to ask for considering their response so far. By the way I've checked my network for any strange devices that could be stealing my data and devices found are known to me.