When something is so unbelievable that the phrase "are you shitting me?" is not enough. It demonstrates that the action taking place, or the event being described is so monstrously horrific that it literally feels like someone is taking a massive shit inside of your chest. Basically, the worst feeling ever is happening, and at this point you're questioning why the fuck.
Douche: LOL I TOTALLY RAPED THIS CHILD WITH A PITCHFORK OF AIDS Person: ...are you shitting inside of me?
Douche: ...No? But I could be.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.