Multiple cells highlighted on mouse click

[mrsm]

I've been using Excel for a long time and have never run into this before. When I click a cell, that cell plus five cells below it are highlighted. It doesn't matter where I click in the worksheet, this is happening. I can move using arrow keys, but this is making it almost impossible to do what I need to do. How do I get rid of this? (I have been using this spreadsheet for several weeks, and this just started happening today.)