Horizontal mouse inverted bug

To explain the title, yesterday I came home to turn GW2 on, my mouse has horizontally inverted itself. (When move it left, it looks right and vice versa.)

This only happens INGAME, when I press ESC to the menu it's 100% fine, other games and normal use of mouse is as it should be, it only happens in GW2 and there's no option to change horizontally invert, only vertically. 

I've nearly put 200 hours into this game and can't remember touching any settings at all, obviously I haven't been able to play since.

Please help me, I've not seen this bug so it's possible it's my end but I don't know what to do to fix it.