When I power it back up, it says "You shut your computer down because of a problem" which is not the case.
But there is a problem or it wouldn't be shutting down. Boot into Safe Mode, put into sleep mode and see if the problem persists. Reboot normally and test again.
NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.
If the problem persists 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 Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report
Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report: