High data usage

Do you have a Photos Library and does it have iCloud Library enabled?

Also download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to include the report in your reply.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

Hi IndyDebbie,

To see network usage from the last time you restarted to present time, go to Finder > Go (in the top menu bar) > Utilities > Activity Monitor. In Activity Monitor, go to the "Network" tab. From there, you will see what is using the internet data the most.

Again, this is only network usage info from the last time you restarted to present time.

Cheers,

Jack

That would be it!

Still run Etrecheck and post the results.