You're pregnant with an Alien chestburster that is getting ready to pop out of your belly...now!
That lump you feel is an unborn fetus twin. Mazal Tov on the new family member.
Probably just an extra organ. Soon you you'll never have to feel guilty about eating for two again.
Your Spleeeeen! (in Invader Zim voice)
They'll still find nothing. Enjoy eating nothing but cabbage juice for the rest of your life.
Nothing. You just really need to fart.