As you move back to get far away from the door you can feel that your truly have no penis as your legs move. Cold air on your crotch makes it feel so clear that you have nothing but a small vagina between your legs.
Your gf walks in. She's cute as always, a petite frame and big blue eyes, she's wearing tight pants though reminding you that she very clearly is packing a cock and balls. The contrast is clear between her small body and smooth soft skin and the masculine bulge in her shorts.
She gasps and says, "oh, sorry, is now not a good time?"
You put your hands over your crotch, feeling your crotch without a dick is pretty strange and unreal. It's pretty clear your girlfriend is erect at this point.