What's eating you?
The meaning is equal to "What's bothering you?".
What's eating her today? She's in a really bad mood.
I don't know. Why don't you ask her the reason?
Refs
- All Clear 3, Helen Kalkstein Fragiadakis
idiom, 14-02-08 12:03, 14-02-08 12:03, Yosei