Early the next morning

The sun is just peaking through Katie's bedroom windows when you wake the next morning. It's early. Too early. Why did you wake up this early?

You hear the cause a split second later. Katie is moaning loudly in her sleep. Squirming around underneath the covers. Clutching at her groin. Murmuring your name. She lets out a short scream in her throat before going rigid, then relaxing with a deep exhale.

She just orgasmed in her sleep, you realize. Has she been doing that all night? You shake your head in disbelief. That's just crazy.

Finding yourself now wide awake, you decide to get up. A quick shower washes the sex and sweat from the night before off your body. You've started keeping a few clothes at Katie's place, so you've at least got a fresh pair of boxers to put on when you finish.

You're out in the kitchen, making a pot of coffee, when you hear the shower start again. Several minutes later, Katie pads naked out into the kitchen.

You nearly drop the mug of coffee in your hands when you see her.

First, the most obvious: her tits have grown several cup sizes overnight. From the firm swell of a B cup to the gorgeous heft of at least a D. Which, frankly, looks absolutely massive on Katie's tiny frame.

The second change is more subtle: the bra and panties have almost completely merged with her flesh. Only a change of color from creamy white to jet black marks where they were, and all the features hidden underneath the material are now visible, from her nipples to her labia lips, still stretched wide around a solid intruder.

“Wow.” It's all you can say. All you can think.

“I know.” Katie groans, sinking gingerly into a chair. “I know.”

“So... that's new.” You remark, taking a seat across from her. “How are you feeling?”

She sighs. “Tired. Very, very tired. And yet, somehow, still fucking horny.”

“How much sleep did you get?” You ask, sympathetically.

She shrugs. “Hard to say. I can't even tell you how many times I orgasmed last night. They all started to run together.”

After the fun you had, you hate to suggest it, but there's only one rational course of action. “Should we, maybe, find a docto-” Your words are cut off by the metallic clank of the mail slot and something hitting the floor underneath.

Katie scowls. “The mail doesn't come until afternoon, usually. Same for packages.”

You get up and wander over to grab it. The package you see makes your blood run cold for a moment, then you dash out into the hall. There's no one out there. No one in the elevator. No one in the stairwell. Where the fuck did they go?

“What is it?” Katie asks, when you come back in.

“Another package.” You respond, staring down at it. The bag is smaller this time and addressed to Katie. But it's otherwise identical.

“What do you mean?”

You sigh. It's time to come clean. “I didn't exactly buy that underwear for you. It just... showed up. Came in the mail, in a bag that looked just like this one.”

She scowls. “Let me see it.” Tearing it open, she looks inside, then smirks. “Well now. Isn't this interesting?” What she pulls out is a black thong. A thong for men.

“No.” You say firmly, already reading her mind. “No. No. No.”

She looks at you in amusement. “Yes.” She hisses devilishly.

“Hell no!” You say again, quickly rising from the table and moving away from the cursed underwear.

Katie simply stares at you expectantly, as if her gaze will wear you down.

“You expect me to put that thing on after the night you just had?”

She smiles seductively. “I'm surprised you're not begging to put it on after the evening we just had.”

“I was just about to take you to a doctor!” You say in exasperation. “We should be figuring out how to get those things off, not putting more on!”

“You're telling me you're not the least bit curious about what this thing would do to you? What it would be like... for both of us?” You swallow hard. She's right, of course. You are curious. Intensely curious.