Im preparing a web-server and have already installed Wordpress on it. Everything worked fine. But instantly, the memory usage went up. Now I only have 300MB of memory free. To check the problem, I disabled some services like php, nginx, fail2ban, psad and mysql. It reduced the memory usage by a small amount but still its too high.

Then I restarted the machine and started working again. Again after some time, the memory usage shot up! How can I locate the exact problem? What might be causing this issue?

PS : Its an Ubuntu

PS : It looks like the swap is not yet used. Swap usage : 0 (The server is a virtual machine (KVM))