Large rectangular box in the middle of my screen

When I opened my laptop today, I noticed a large rectangular box in the middle of my screen. It goes from the top of the screen to the bottom and is significantly lighter. It's not visible on backgrounds that are very light, however, it's very noticeable on backgrounds that are dark. When I tried screenshotting it, the rectangle was nowhere to be seen. What is this, and why did it happen out of the blue?

A few minutes later, I noticed it was slowly starting to fade away. It's still very odd though.