Excel workbook automatically deleting or merging new sheets after saving?

I have an Excel workbook with one sheet full of a raw data output ("RAW"). Today, I tried adding another sheet for the formatted data ("Cleaned"). Everything seemed fine until I saved and closed the workbook; I saw that the file size went from like 150 KB to 7 KB. After opening it again, all of my raw data is gone, and all the content of the new sheet I made ("Cleaned"), minus formatting, is now in the "RAW" sheet.

I reverted to an earlier version and re-created the "Cleaned" data sheet, saving it and closing the workbook, and the same thing happened again. What's going on? Every edit I make, regardless of how small (i.e. putting a "1" into a new sheet, then saving and exiting) leads to the content of the new sheet replacing the old sheet under the old sheet's name. It'd be a real tragedy if I lost any data, so I'm a bit afraid to open my other workbooks. Any help would be really appreciated.