Increase in memory usage when site stayed the same?
Thanks for posting your question here!
If the memory usage increase seems to go away and then occasionally re-appear, it is probably being caused by the WP-CRON system running its tasks. You can install a plugin like WP Crontrol to view and manage WP-CRON tasks.
If the increase is constant, then you should contact your hosting company. It's possible they made a change in your hosting plan without telling you. If you have a VPS or dedicated server, take a look at its update changelog. It's possible your server did an update that caused the increase.
Also keep in mind that some plugins will update automatically in the background, so you might want to take a look at the plugin files using FTP. If you see any plugin files with recent last-modified timestamps, then that plugin updated itself without your knowledge. (I personally would deactivate and delete a plugin that did such a thing.)