How can i find where my node.js app crashes?

Here's the thing: I created a node.js app and mongodb and deployed on Amazon EC2.

By crashed I mean: It wasn't possible to access the app.

Node.js Server -> Nginx, EC2 m3.large, PM2 module with cluster (2 vCPU)

Mongodb Server -> EC2 t2.micro

My problem: Yesterday, the app reached 600 concurrent connections, then it crashed. When I checked pm2 status, both CPUs app processing was 1.50gb

Running pm2 restart all , 2minutes later everything was working again.

Question(s):

Where is the best point to start digging? Mongodb max connections ? Nginx ? Node.js app handling connections wrongly ? EC2 servers not enough CPU ?

I really need some help here. Thanks !