The meaning
of the idiom what's eating (someone) is making a
person angry or upset. It often appears in the form of a question, as in “what's eating you?” But it can also be used in other
ways. It is related to the idiom eating away at, where eat means bother or make
angry.
Example
#1: What's
eating you, anyway? I do everything I can to please you, and you're
still in a bad mood and angry at the world for some reason.
Example #2: I don't know what was
eating him, but all of a sudden he started yelling at the players and
the fans and stomped off the basketball court.
Example
#3: If you really want to find out what's eating her, just ask—sometimes a direct
approach is much better than guessing.
Example
#4: I felt mad, anxious, fearful, and
sad, and I wasn't sure what was eating me—so I
made an appointment with my doctor to figure it out.