Does home networking via my router go against the Xfinity data cap of 1.2TB??

If I use my router as a home network to transfer between my home computers does that count against my 1.2TB data cap? I am asking this because about 2 1/2 months ago I connected my router to an ehternet switch, connected that switch to 2 of my desktop PCs in order to easily transfer videos, pics, files, etc between them and also occassionally with a laptop via wifi connection. During the first month my data went slightly over the 1.2TB data cap (using the courtesy overage allowed for no fee), which I just shrugged off as downloads for upgrading my home PCs for better compatibility. But then, the 2nd month I went WAY OVER the 1.2TB data and got charged an extra $60 in overage fees! So, I slowed down my normal downloads for games, videos, streaming, etc for this month to make sure that I didn't go over again. Then today I received a text notification that I have used 90% of my data cap and it's only the 15th of March... halfway through the month! There is no way that I used that much data unless Xfinity is counting data transferred between my own home PCs. Does anyone have info if this is correct? Or how I can use home networking without affecting my data cap? Otherwise my data stream is being hacked or something because it is only me and my wife using it and she does nothing but stream and use her cell phone to surf the internet, both of which use very little data at all. Any help or clarification would be greatly appreciated.

Thanks in advance for any responses or help with this!