The vaguely pleasant warm feeling returns to your chest, then it feels like it's trickling down the back of your spine. You squirm a little then feel the strangest stretching feeling and your pants tighten. It feels like you've put your hand down the back of your pants.
You squirm around to try to see what's going on. There's something in the back of your pants, some thick furry tube pressed between your pants seat and your butt cheeks. And it's growing.
You look around quickly, see no one, unbuckle your pants, and drop them and your boxers to your knees.
The tail extends out like a third arm, a real weight pulling at your spine, standing out a foot, then two feet, and then three feet out. It lazily wags as you squirm to look at it. You finally grab it with one hand and nearly freak out because this is the first time you've felt something grab your TAIL.
You have a tail, covered in tabby cat fur. You recognize the color and shout out "The costume!"