Do you have any event procedures taking place?
Right Click the Sheet Tab, click View Code...
If the VBA editor page that opens is blank, you have no worksheet event code.
You can also check for WorkBook event code.
In the VBA editor, double-click the "Thisworkbook" module.
If the VBA editor page that opens is blank, you have no workbook event code.
To open the "Thisworkbook" module in version prior to Excel 2007;
Right Click the Excel Icon to the left of the File Menu, click View Code...