It's vignetting, most likely emphasized by the CPL. A lot of wide angle lenses produce some vignetting to a different degree, most prominently with aperture being wide open. If you read lens reviews you'll see it in the test details.
Vignetting usually becomes less obvious if you stop down the aperture a bit. At f/5.6 and smaller it should not be an issue anymore.
Did you stack the CPL over an existing UV filter? Try using just the CPL without the UV if you have one.
If the CPL alone is still too thick and creates vignetting, then try using a larger CPL with a step up ring. Step up rings tend to be slimmer and using a CPL that is larger will help to reduce vignetting. More expensive though.