For me it depends on the problem. If I am having problems with C it like a too full about to pop pressure kind of pain. With a side of sharp stabbing where it feels like my guts are trying to split open. With D I go anywhere from the mild discomfort of little tiny baby spasms that go away when I rub my tummy in the right direction to burning, stabbing, oh my lord I'm going to die type pain. It some days it the fire of burning lava, other days it's a weird burning like holding a too cold ice cube. I have so many different types of pain with this that I can't say that I really get one kind of pain or another.
As to the alien thing, I see it as I have a colony of gremlins living in my guts and they like to hold ball room dance lessons while wearing golf shoes. Some days they wear the extra pointy kind.