Trying to understand data usage

I have AT&T internet only.  No cable, Direct TV or wireless phone (phone is with another carrier.)

I use my wireless to watch Netflix on a Samsung Smart TV.

If I watch a lot of Netflix, my usage will be up, which is understandable. Herein lies the problem:

One day I hadn't watched and Netflix and as soon as I turned my computer on, I showed a usage of over 6gb !

Is my phone downloading data at night ? I have called AT&T twice and haven't been able to get any intelligible answer. And techies here that can explain this ? Is data usage calculated from midnight - midnight ?

Thanks in advance !

Dave