It's a DDoS.
Meanwhile in the cloudflare website :
Under DDoS Attack? We Can Protect You in an Emergency
Cloudflare mitigates DDoS attacks, including those that target UDP and ICMP protocols, SYN/ACK, DNS and NTP amplification and Layer 7 attacks. Our global network has defended against sustained attacks over 400Gbps. If you're under DDoS attack, we can get your site back online within minutes.
Yeah, within few minutes...
Activate DDoS Protection
Get These Benefits Within a Few Minutes
Protection against DDoS attacks of all forms and sizes
24/7/365 support backed by our team of security experts
Predictable, flat pricing regardless of attack size or duration
Use of Anycast for both DNS and TCP
All DDoS attacks absorbed before they overload your server
Mitigates both app-based and volumetric attacks
Automatic learning from network effect of 2M+ websites
Uptime guarantee
Global, distributed network with 120 data centers
Legitimate traffic can still access your content
Since cloudflare implemented we experienced less ddos attacks, i don't know whether cloudflare is doing a good job or the attackers getting lazy... but for sure CF isn't the perfect solution.