Cloudflare is a CDN service provider. CDN stands for content delivery network. In CDN your content copy is distributed among different location. Content is served to customer on the basis of geographic location of user. Nearest server to user will handle request. In this way your website load time reduced. It also acts as an intermediary between actual server, also known as the origin, and its end users or clients.
SSL Setting of Cloudflare:-
Flexible SSL – If your website don’t have SSL. Using flexible SSL, content is served securely between cloudflare and user. But connection is unsecured between your server and cloudflare.
Full SSL:- For this setting you need SSL at your server. Secure connection between your visitor and CloudFlare, and secure connection between CloudFlare and your web server
Full SSL (Strict):- For this setting you need a valid SSL for your website. Secure connection between user, cloudflare and your website.
Probability of getting infinite loop of your website – You are using flexible SSL but your website already have a valid SSL or vice-versa
If you already have a SSL just use Full SSL (Strict) setting, your problem will be solved.