Before your sexy afterglow had even worn off, a firm jolt took hold of your whole back. Its like something was trying to stretch and shift your whole spinal column at once. You dizzily wobble around in your seated position. Another jolt, and all at once your sealed-up bottom didn't feel so small and empty anymore. It felt like your backbone was trying to divorce you too, trying tearing itself out of your rear like the parasite in that alien move. You kind of wanted it to leave too, because the pressure was starting to get unbearable. You clutched your own belly, clenching your teeth during the build-up to the next jolt, and gasped in relief when you heard something finally starting to rip off your bare skin. Not having the willpower to look back and check on this, you doubled over to wait for the next jolt. SLAP! You felt your backside collide into the chamber's back wall, sending a vibration all the way through your spine even up to your skull. You looked back and this time you saw it--over a foot of scaly length sticking out of a tear where your butt used to be. You squirmed wildly as another jolt nearly doubled the thing in length. Then you flinched as a spasm surged all the way up from the faraway end of your spine.
Left panting from basically giving birth to half a snake basically, a torrent of new impulses started swirling within your mental awareness, and they were all too new for you to resist any of them. You felt your scaly worm-like extension starting to slap against the wall again, and the floor, and your own back, and occasionally it would just feel weightless, like it was hurtling through the air. You looked back again to watch your tail explore its full range of movement. You wanted to stop it from thrashing, because the slapping noises sounded painful and some of the impacts really did hurt a little bit. Its nerves and muscles (YOUR nerves and muscles!) were too busy trying to learn how to work together, though. You only managed to adjust the force at first, so that you didn't pound the thing too hard against anything and hurt it, or at least not as frequently anyway.
Starting to make sense of the feelings coming back from the tail and your view of how it was moving, you started to get a more usable sense of cause and effect from your new appendage. You cause it to flex it behind you, and you even figured out how to make it sit still. You could curl it around toward the front of yourself if you wanted or send s-shaped undulations through the length of it like a garden hose. It was like you had a new employee of your own, one who'd had a rough first day on the job but was now ready to do what you wanted. It was time to announce a workplace inspection: you instructed your new partner to present itself for closer examination immediately, raising its distant end-point high up in front of your face so you could see it. You almost wanted to thank your tail when it obeyed. Cautiously reaching out one hand, and noticing that it didn't flinch or try to run away, you gently cupped your fingers around it, widened your eyes, and tentatively attempted a slightly stronger grip.
Allowing your tail to relax in your and rest itself in the grip of your own hand, that's when it finally sank in that it was really you down there, all the way down to the scaly thin tip. It looked like you were holding onto a snake, and you were. But this snake was you, and you were the snake (or at least, you were going to be). You felt like you had coaxed a powerful entity into your own grasp--you had, because it was part of your own being, not just something that was stuck to you and had to power to cause you pain. You didn't just HAVE something a snake has, you WERE something a snake IS because you were really and truly going to be a snake for the rest of your natural life ("for better or for worse," as they said at your doomed wedding). You relaxed in its serpentine, satisfied that you were the one touching YOURSELF, at both unnatural points of contact.
Soon you had both of your hands gingerly exploring your tail, feeling its cool scales on your fingers, enjoying the heat of your warm hands against it, holding yourself in your lap. Looking down at your dangling legs, seeing that they didn't have toenails anymore and your toes had mostly shrunk into your paddle-shaped feet, you realized you were really holding on to your new means of getting around. And what was really unnatural about touching yourself this way really wasn't your tail against you, that was only less familiar. Now, the fact that you had hands to work with was an incompleteness that the nanobots were at some point going to fix. If you thought being a pet meant never having to lift a finger, your snake transformation was going to end up taking that to a whole new level.
"Damn fool looks like he's trying to jack of his tail now," you heard the old man musing with an eyeroll. You wonder if the pet store knew they didn't have very good soundproofing, or if they just didn't care.