MacBook Air :: Losing Saved Changes In Excel & Word

I just migrated from Lion on an old MacBook series 2007 to a MacBook Air.  Yesterday, I opened up a grading file in Excel 2011, entered grades, saved the file (NOT Save As - just Save) handed back the assignments to my students - and then went to reopen the file later, and all of my changes were gone.  I made more changes, Saved, just opened the file again, and they aren't there.  To experiment, I opened the file just now, wrote in a silly comment on the top line, saved, reopened without quitting excel - and the changes aren't there.

A temporary work-around is that I can do a Save As every time I make changes, but that's ridiculous - I'm often updating a file and need the changes to be there. I just tried this with Word as well and I'm having the same problem - some Saved changes are there, some are not.