According to: https://help.gnome.org/users/gnome-help/stable/screen-shot-record.html.en
We can record our screens using Ctrl+Alt+Shift+R, works very well. But I have 2 screens or monitors. When recording, the software captures both screens, but I only need to capture one.
How can I record only one screen?
Install Kazam, it's a great tool to record single/multiple screens
Install Kazam in Ubuntu
If you want to install the newer Kazam 1.5.3, you can use unofficial PPA that is available for Ubuntu 18.04 and 16.04:
You also need to install a few libraries in order to record the mouse clicks and keyboard presses.
Source here
Or you can simply temporary turn off second screen.