What's eating you?
 →  무슨고민있어?

[예문]
A:Cheer up! You haven't said a word all morning.What's eating you?
 힘좀내!아침내내 말한마디없잖아.무슨고민있어?

B:Nothing, I just have a sore throat.
 아니,목이 부어서그래.

A:Oh, I thought you were angry about something.
  난 화가난줄알았지.