What’s up? Nothing. Why is it, when it’s someone else, it’s always nothing, and when it’s me, it’s always something?

When you are suspected of doing something that you might have done, but it’s nowhere near as bad as what others do all the time and got away with it - and you are lost for words :)

https://www.urbandictionary.com/define.php?term=What's up

what's up

A casual greeting or conversation starter, somewhat of a rhetorical question.
Usually followed by something like "Not much, you?," "Nothing much," or even answered with a "Hey man, how's it going?"
syn: How's it going? What is new?

A question that means "hi" or "What are you doing"
It can get annoying, but people don't seem to understand that.
The best way to answer is "I think it's the opposite of down" if you are annoyed by this question