Unusual Private Inbound Traffic and CPU/Disk Spike (php5-fpm, nginx, DigitalOcean)

I got a couple of 504 nginx server errors yesterday. After checking the graphs of my droplet on DigitalOcean I noticed that there is a unusual high peak in both private inbound traffic and CPU/Disk usage.

Comparing it with the last 30 days this has never happened before. So it can't be a backup process or anything.

Whats weird that its "private inbound" and not public.

I haven't made any changes to the configuration in the past 7 days. Private networking is enabled because I am running mySQL on another droplet, apart from this one that runs nginx with wordpress.

What can I do to find out whats happening?

Running top it looks like php5-fpm is causing the high CPU usage. I have already restarted both nginx and php5-fpm and as well rebooted the entire droplet but it doesn't changed anything. I saw this topic that has a similar problem but came to no conclusion.