Ports 80 and 443 problem

Hi,

I have noticed that opening these two ports on my router for self hosting Nodejs apps (Website and API) using Nginx makes my internet drop multiple times a day! I use Cloudflare as a proxy yet I feel like I am getting DDoSed regularly.

Of course when closing these two ports I instantly do not have that problem anymore.

Is there something else I can do to open these ports but not getting exposed like that ?

Thank you