Step 1: On your cPanel (or control panel) server, do a dig for your localhost:
dig www.mydomain.com @127.0.0.1
Do you get a response that says www.mydomain.com.cdn.cloudflare.net? If yes, go to Step 2.
If at any point you do not get the ".cdn.cloudflare.net" output, this is the step where the DNS is not updating properly.
Less than 24 hours since you enabled Cloudflare: If it has been less than 24 hours since you enabled Cloudflare, then the issue is likely that the DNS is still updating and has not propagated everywhere.
Over 24 hours since you enabled Cloudflare: If it has been more than 24 hours since you enabled Cloudflare, then the issue is the cPanel or control panel instance is not the authoritative name server for your zone. To troubleshoot this, you'll have to refer to your control panel documentation. You know where the issue lies based on what step that the DNS is no longer returning a “.cdn.cloudflare.net” output.