Node is flagged, node author comes back to the site, how to show how many new flags since last visit? I created a "new activity feed" view page in which a user can see how many new comments have been posted to his own nodes since his last visit 

Eg : since you last last visit :
- 21 new comments have been published on your node #title, in a total of 65 comments
- 3 new comments have been published on your node #title2, in a total of 8 comments

The nodes can also be flagged by other users.

Using flag module and views, it is possible to get the total amount of flags a node has received at any time.  

I would like to be able to show the node author, how many "new" flags each of his nodes has received since his last visit.   

Any help is welcome
To answer your posted question (title), you'll need to extend functionality. 

By default, the core comment module &amp; flag module have no default mechanism to store comment/flag view history. You would need to do something like:


Build something like the Drupal core history module that tracks a user's comment/flag views 
Build a simple aggregate comment/flag query based on a user's last login/access time
...or maybe find a module that offers this functionality (not sure it exists and module recommendations are off topic here)