Okay ... enjoy your painkillers, man.

Incoming vibe killer: no, farts do not count as negative mass. They do in fact have positive mass. You will weigh ever so slightly less after farting. And, in theory, if you were in a frictionless environment, you could propel yourself by farting, because the fart's mass would act as a reaction mass and propel you like a (very weak and stinky) rocket.

Negative mass would be a very weird thing that breaks physics in a lot of ways, and probably isn't physically possible in the first place. Allowing faster-than-light travel is only one of many stupid ways negative mass would horribly wreck all known physics.