Are you behind a load balancer such as cloudflare?
The issue there is that they terminate the https session before it hits your web server so Laravel thinks the user is requesting http:// and makes links using http
There are several threads on the forum about this problem. Just search for cloud flare
edit: duh moment, I see herokuapp.com in your url. Could be the same issue though.
Cronix, thank you for your answer! I added the code to the boot method and changed the APP_URL, however when I redirected all HTTP to HTTPS by modifying the .htaccess file I was getting an error in my browser ERR_TOO_MANY_REDIRECTS so I didn't change that, but it still works now with the other suggestions!