If you open your cpanel and wp-admin ,then use the code editor function to read the error_log to see exactly where the memory went.It will even give you the “line” in the php file that is causing the fatal error. However,since most such fatal errors are inconsistent ,a combination of other memory hogs may have triggered an error in that line. One such memory hog is a Google+ plugin and another may be the loading of sprites or ad images that are not served from a CDN nor from a Google server.