When you have been having a bad day all day, you got up on the wrong side of the bed. Example: "What's wrong with her today?" Reply: "Oh, she just got up on the wrong side of the bed."

Sometimes we get a bad start to our day, it continues throughout the day, and people notice. Example: "I told you to pick up your things! And don't play your music so loudly!" Reply: "Wow. It looks like someone got up on the wrong side of the bed!"

It is as if we started our day ("got up") in the wrong way ("on the wrong side of the bed") and that has affected everything else that happened since. Example: "Don't start yelling at me just because you got up on the wrong side of the bed."