You try to put your weight on one leg to pull the other out, but no such luck. The one leg sinks deeper in and hits something solid.
You look down, confused and crooked, and feel the solid thing move.
Suddenly, you feel several sharp objects stab your leg. You grit your teeth and try to recoil out of the mud. Luckily, you put enough force into it to pull yourself out of the swamp.
Hastily, you run away from the swamp and away from whatever was in it. Soon enough, you get back to the forest.
But you soon realize that this is a different part of the forest than you were in earlier. It gets harder to recognize anything.
Soon, your arm starts to itch. You scratch and feel eerie, rough skin. You look at it and see that your arm has started to change in color and texture, becoming dark green and scaly.
You stop dead in your tracks as you see the creepy scales spread up and down your arm, your hand growing larger, sprouting claws instead of fingernails. Your forearm starts to bulge, you bicep expanding, straining your sleeve.
Your sleeve suddenly rips apart, revealing a massivly muscular arm, green and scaly. The scales keep spreading, up your shoulder, bulging those muscles, ripping your shirt further. The same begins to happen on your other arm, muscles growing, scales spreading, hand becoming monstrous.
Your chest comes next, muscles filling out like an evil bodybuilder. Then, an unexpected thing happens. Two orbs begin to bulge out of your muscular chest further, stretching your shirt still more.
You first think your mutating even crazier, but as the orbs grow bigger, they jiggle a bit. You poke them with your creepy claw and are surprised to find that, even though they are scaly like the rest of your torso, they are soft.
With horror in your mind, you realize that they are breasts.
And they're big.
Your torso and arms continue to bulge (along with your newfound boobs) as the transformation spreads to your abs, growing into a thick six pack, then a thicker eight pack, then a humongous ten pack.
Suddenly, your shoes feel tight, and your pants grow taut all around. Your pants and shoes seem to grow a bit with your muscular legs and feet, and then tear into shreds, showcasing a gigantic lower half, scaly, of course, ending with big ol' sharp, clawed monster feet.
Somehow, your underpants have held up, but not for long, as you feel something else start to sprout out from your lower back.
A tail suddenly bursts forth, tearing a hole into your undergarment. You expect this to be the end of your poor boxers, when you feel a bit of the front slacken a bit. It doesn't take a genius, along with your still-growing bust, to figure out that you've lost your penis.
Your body still gains more muscle mass, and more mammary mass, growing taller and wider. You feel your neck grow thicker and you know that your face is next.
Your nose grows wider, than forward, dragging your mouth along. Slowly, you get a muzzle, long and powerful, with sharp teeth growing out the top and bottom of your extended jaw.
Your entire head begins to grow wider, your vison becoming odd as your eyes move to the side of your head. Your ears disappear, replaced with holes on the side of your head.
Oddly enough, though, your hair starts to grow longer, getting silkier, and more feminine, hardly a match for your muscular reptile body. It stops ant waist length, which is pretty long by now.
Your muscles and breasts finally begin to slow down, then come to a stop. Your chest is so large and builging out of the small still of cloth that used to be your shirt, you can't see directly in front of you.
You notice that your much taller than you were before, several feet taller. Tall enough to be considered a monster.
Along with your green, scaly skin and your sharp-toothed muzzle, you figure out what bit you in the swamp.
A gator.
Presumably female.