This sounds like you changed the resolution setting in Export - Advanced from what the original Video Mode is. To reset, when going to Export, click Reset to have Export match your video mode’s resolution.
It is highly possible there is some other reason, but without more information it will be hard to keep guessing.
It will be helpful to get a screenshot of your project, with the clip selected. Playhead should be on that clip showing in the preview, full view of the timeline with all tracks visible. Full view of the filters pane for that clip, specifically showing filter in question.