Deep within your reshaped body, where the wish reassigned your body, reshaped your balls into ovaries and formed a vaginal tract and womb, your girlfriend's semen has been pumped into you, and as you rest it is preparing to leave its mark. When your sex was adjusted your many sperm were bound into a vast mass and transformed into a single ovum, remade not to fertilize but to be fertilized.
Likewise, your girlfriend's ovaries were converted to produce masses of sperm, which are now inside you thanks to your lovemaking. His strong, energetic seed tirelessly chews away at your egg's walls, both desperately following their newfound genetic instincts just as you and your girlfriend did.
At last a sperm weans its way inside the egg, burrowing to its core and fertilizing it. The newly-fertilized zygote embeds itself in your womb. You have become pregnant.
Right now, obviously you don't know this, since you're asleep. But when you wake up...