It began with an unpleasant feeling in your core. You could feel yourself warming up, and an ache in your extremities before you felt yourself shrinking. Your hands regressed from their well-formed digits to smaller, webbed ones. Your legs and head shrunk, and while you were happier for the extra room, the unusual change distressed you, and you kicked and squirmed as much as you could against the shrinking walls of the uterus you lived in.
But just as suddenly as it began, the regressing ceased. There was a light that came from below, and when it touched you, you ceased shrinking, and began reforming - into something different. Your hands began to grow, back into usable digits, and your fingers ended in slightly pointed nails. Your skin began to tint into a pale shade of lavender, and several darker winding shapes adorned your body like tattoos. Your legs were growing back too, but as they got close to their original size, they fused and grew, lengthening and coiling around your body, refilling the space your body once occupied as smooth scales formed along its length.
But the changes were more than cosmetic. Where before you were only a barely sentient fetus, your mind began to develop into true consciousness, and knowledge flowed into you seemingly from nowhere. You began to slowly grasp basic concepts - life, warm, cold, talk, move - and then began to understand greater things - language, personality, purpose, species. Within a matter of minutes, you knew everything that had just transpired.
You are a baby, formerly a human child, but now, a baby monster girl. An echidna. One of your hands reaches to your belly, and the other to the tip of your snake-like tail. With one hand, your rub along your tail's length, feeling the scales all the way up to your rear, joining up to your hips. The other hand rubs your tummy and grabs onto the umbilical cord extending from your belly, connecting to a placenta attached to your mom's inner walls. Still holding on, you wiggle it around, watching it swish through the amniotic fluid. Yes... that's what it's called. You know that, and try to concentrate, to try and find out more of what you know.
You were sired by a book... "The Monster Book of Impregnation". Well, technically, you were sired by someone else, but the book is what made you what you are, and could make you from scratch if needed. It was a magical artifact, linked to a different world, and a tool of one of its greater beings - a pseudo-deity of fertility whom you note was also an Echidna, and the progenitor of many species of that world. The book could manipulate space and time to a degree in order to suit its needs and continue its existence, but in a creature's hands, its primary use was as a fertility aid... with a 100% success rate.
As it all sunk in, you found yourself getting excited, what it meant to be alive, what you could do in the world. You were happy just to be here, and let out a punch at your mom's walls while simultaneously wiggling your little snake hips. You struck out with another and another, and heard a voice say "Alright, alright! I get it. I'll get something to eat." You feel a hand resting on the outside of the womb, and you give it one more strike. Pausing for a moment, you realize you weren't really hungry before, but now, you're starving. Transformation must've taken it out of you.
You feel movement, and a jostling as your mom starts to move. She must have been sitting down. There's a faint sound of slippers shuffling across linoleum, and the occasional clink of cutlery as she sets to work making something to eat. When she does sit back down on the couch, you're squished a bit and give an indignant wiggle, but the taste of a PB&J sandwich relaxes you a bit. The two of you eat mostly in silence, but when she's done eating, your mom gently rubs her belly and picks up the book.
You can tell, even inside her belly, that it's the Monster Book of Impregnation, and that something's probably about to happen. The question is: what?