it's not usual at all maybe your site is under attack,
Increase the security ASAP, first of all, check your activated theme functions.php file. Maybe you can find some miscellaneous code there if you find then delete it. Maybe you were using any nulled theme or plugin.
Now check you're all files and folders permission,
- All files should be owned by the actual user's account, not the user account used for the httpd process.
- Group ownership is irrelevant unless there are specific group requirements for the web-server process permissions checking. This is not usually the case.
- All directories should be 755 or 750.
- All files should be 644 or 640. Exception: wp-config.php should be 440 or 400 to prevent other users on the server from reading it.
- No directories should ever be given 777, even upload directories. Since the PHP process is running as the owner of the files, it gets the owners permissions and can write to even a 755 directory.
Also, protect your wp-content folder.
You can also install https://wordpress.org/plugins/sucuri-scanner/ for WordPress free security plugin and try to scan your complete.
I'm not sure but maybe there can be some scheduled post :D, check your Cron jobs or disable them and then check again.