You may have multiple windows open with different views of your plan.
To open a new view window press the V key.
You cannot edit in View windows. However, you may adjust the view using mouse shortcuts for orbit, zoom and pan. See View> Adjust View for more information.
To swap the View and Main windows double-click in the View window.
Use mouse shortcuts for orbit, zoom and pan to adjust View window view. Of course, you may also adjust the view in the Main window and swap it with a view window.
The more Views open the slower rendering and editing will be. Each new view requires rendering the scene again so closing view windows will improve rendering speeds significantly.
Also see Edit> Improve Edit Speed for other ways to up performance.