Excel 2013. Problem with locked cell

If I enter data in one specific row, ROW 11, in any cell in this row, the data automatically also appears in cell D11.For example: enter data in cell A11, the data appears in A11 and in D11. Enter data in H11, the data appears in H11 and D11.
D11 will also not accept a formula. If I enter the formula =C11-A11, the formula appears, not the result of the formula.

I have tried clearing formatting rules in "Conditional Formatting" but with no success.
What did I do to cause this?
How can I undo it?

Is there any code in the workbook?

What's the file extension?