so I have a laptop and another monitor, whose position was to the left, and then I positioned it to my laptop's right.
The problem was the mouse translation from laptop to the monitor, annoyance !
So I changed settings, and I got an error when trying to apply the new position.
Solution is given here:
http://askubuntu.com/questions/174195/gdbus-error-when-trying-to-enable-second-screen
Which led me to issue locally:
> xrandr --current
and then the proper command for my case:
> xrandr --output LVDS1 --auto --left-of HDMI1
and bingo, things are back to normal.
More info for fun: http://www.thinkwiki.org/wiki/Xorg_RandR_1.2