@darknova It's not because you have upgraded wp to latest version.
Its because of the following reasons:-
You have too many visitors and over load free server resources.
This might be caused by crawlers crawling your site; use robots.txt to limit the crawlers.
If you are using WordPress with lot of plugins your site might be under attack, remove all plugins only keep the one that you are really need.
If you have many add-on installed, your site take too much resources to run.
Your script is taking to much server resources to run.
Migrate to new panel to avoid such issues.