You notice you're starting to feel weird in your belly. A hard bony ridge seems to be forming just under your rib cage, with a second one under that. A sudden shift down there makes you realize you can move the ridges apart and back together, creating a very weird stretching and pressing near your insides. You feel an odd emptiness down there, like some sort of cavity is forming, and then you feel a wetness start to form inside it. Something muscular starts shifting around inside it, which you learn to direct under your own volition somewhat. There's a swollen mass you can move in 3 dimensions around the inside of the cavity.
After a moment of focusing on whatever's forming down there, you notice that you've stopped swallowing for some reason. Your mouth is getting very dry, the discomfort associated with such dryness is starting to pass. You realize you can't even figure out how to make yourself go through the motions of swallowing anything, as much as you try experimenting with various tongue and throat movements in the back of your mouth. Could this have something to do with your transformation? What are the aliens doing to you? Or are you just psyching yourself out since you can't actually see anything happening from outside you.
Something long slithers its way through you, starting upright near your neck and dangling itself forward through the front of your chest cavity. You could almost swear you feel it tightening or shortening itself, and then it attaches itself somewhere near the back of your new air pocket, which is so wet now that it feels more like a second bladder. All at once, a surprisingly familiar impulse occurs to you and your muscular growth shoves the water toward the back of your pocket, where relief valve opens and drains the water out, pulling the water back irreversibly into whatever and muscular tube you felt repositioning itself earlier. More moisture starts to form, and repeating the same action you almost involuntarily drain it back out. You know what this feels like, but you should be feeling it down there.
A small indentation starts to form at the top of your belly, turning into a thin line that seems to redden the flesh around it. Before you have time to speculate on what it might be, you clearly recognize the completion of fully formed lips in the middle of your torso. Your head-mouth gasps, still having access to your lungs from up there. Your lower mouth swallows nervously. You try to get the thing open to see inside it, but before you can get your hands in position to force the lips apart, they open up all on their own in response to your wish. Its as easy as opening your first mouth! You marvel at a moment that you can only swallow and salivate using that mouth, but you only seem to be able to talk through your top mouth.
"What are you doing to me?" you ask the aliens through your head-mouth.
"Turning you into a different species," they say. "One from a planet eighty light years from here. Not every animal keeps its feeding apparatus so far from its stomach, you know, nor do they all eat and vocalize through the same opening."
You're starting to wonder what you may have gotten yourself into.