what's eating?

What's eating someone?

Inf. Fig. What is bothering someone? Tom: Go away! Bob: Gee, Tom, what's eating you? Bill: Tom's so grouchy lately. What's eating him? Bob: Beats me!

what's eating somebody?

  (informal)
something that you ask when someone is angry and you want to know why He suddenly noticed I wasn't joining in the conversation. 'What's eating you tonight?' he asked.