I would take a look at your web server access logs. Are you getting tons of requests? What IPs are they coming from? If you haven’t already, you should implement a firewall rule at your origin to block any requests not coming from Cloudflare IP ranges.
That means Cloudflare was able to connect to your origin, but it did not receive a response within 100 seconds (default). This is usually caused by high load at the origin - it also looks like it is the case here.