Close Lightroom and all other apps, shutdown your system, restart system, and try again.
After restart, it is the same again, memory goes up.....
I suggest rolling back to LR 6.4.
1) Uninstall LR 6.5
2) Download and install the 'Application Installer' at the above link.
3) Download and install the LR 6.4 Update at the above link.
If the issue persists with LR 6.4 there is some other issue.
Please update Lightroom to its latest version 6.9/2015.9, it has included the fix for Lightroom 6.8: Memory Leak | Photoshop Family Customer Community