I can't play full screen when I play split screen on COD Black Ops, help?

I know that with split screen each person gets half the screen, but now there are 2 black margins between my screen. I went to game options & the arrows are where they should be. It's usually full screen. My tv is a Samsung 42' 450 series. The black margins only show up whenever I'm playing multiplayer split screen! What could be wrong? Is it the Xbox? The actuall COD Black OPS game? Or my tv? I even rest the tv picture back to factory settings. Everything was fine until today. Please help me! I have an Xbox 360 slim & use an HDMI cable.Thanks.