As already
@Motto73 said, It seems a scaling-related problem.
I've never seen something like that but I guess it's something related to the "half pixel" problem.
Sometimes can happen that with very low resolution screens with even lower resolution played games the images in screen cannot be represented properly, this will make the program "guess" what could be the likely final aspect of the scaling process, this will result in image deformations more or less evident.
Here you can deepen the subject.
Can you please let we know what's your current game resolution? Can you also try another one (in full screen mode) and post the result in order to see if it still happens?