Data usage doubled in one month

I have been with Xfinity for over 10 years and never exceeded the 1.2 terabyte limit. This month Xfinity advised me that I was close to going over the limit. I called support twice and all I got were Asian reps that just kept reading off a screen and insisted that I upgrade to unlimited. For the last 3 days I shut the modem off and paused all connected devices. Our phones are on the T-Mobile network. The data usage on the app showed I used 11 gigs. My average use is 500 to 600 gigs per month. Should I change my password?