who pissed in your Cheerios?

Who pissed in your Cheerios? Is a sarcastic idiom phrased as a question which is usually said in response to someone who seems to be in a very bad mood, as if they might be in a bad mood because someone literally urinated into his bowl of cereal in the morning. Of course this is in no way a literal phrase, but a way to ask why they were in such a bad mood, as in what could have happened to cause it.