Hi there! I can definitely help you with that. It sounds like you might have accidentally turned on the "Extend Selection" feature in Excel. Here's how you can turn it off:

1. Open Excel and click on the "File" tab in the top left corner.
2. Click on "Options" in the left-hand menu.
3. In the Excel Options window, click on "Advanced" in the left-hand menu.
4. Scroll down to the "Editing options" section.
5. Make sure the box next to "Extend data range formats and formulas" is unchecked.
6. Click "OK" to save your changes.

That should do the trick!