Screen resolution

When i started playing cf, screen was just the size of my pc screen, so just as its supposed to be. but since yesterday, i have changed no options, but my screen now is square...
My usual desktop resolution is 1366x768, but now my cf resolution is 1024x768. changing it at options doesnt help, but it keeps having black on the left and right side of the game, which makes the game thinner, which makes my enemys thinner, which makes it hard to aim and not really anymore fun to play(it also ruins the graphics) Do you have any solution? i googled for solutions already, but only found someone that had exactly the different from me, he wanted black on the sides, but nobody had a solution for him. Please help me, i would appreciate,