What do you mean exactly by “changed the nameserver”?
To manage your domain on Cloudflare you need to set the nameservers at your registrar to those provided by Cloudflare (*.ns.cloudflare.com).
Once you’ve done that all your subdomain management will be done by Cloudflare. If you want Cloudflare to provision SSL certificates for you then you need to make traffic pass through Cloudflare by enabling the orange-cloud proxy setting under your DNS.
Make sure that SSL is set to Flexible or Full in your SSL settings.
Maybe drop your domain in this thread so others can have a look at things and try help?