unusual disk activity
Open a terminal and run this command:

 sudo fs_usage


It will dump the filesystem activity in real time. Once you're ready to stop the trace, close the window or press Control-C. If you're not familiar with sudo or fs_usage, right click on them before you press return and open the manual pages for each command to see what they do.

It's almost certainly fine, but without seeing the scale of the graph, it's hard to know for sure. Your Mac can take several days to optimize the files needed in the fast portion of the drive and finish indexing things (if you dump a lot of data on the drive and spotlight can't index it as it's written).