...slowly but surely growing a tail.
Your girlfriend's gone a bit quiet as this tail starts to grow. Her spine shifts, as the small nub starts to get longer. The skin surrounding the area starts to sprout out a few small hairs. White hairs, apparently. As the nub got slightly bigger, the hairs above her bottom grew thicker and soon coated that nub, and then shaped the fur surrounding the tail into a soft serve ice cream-esque shape. A rabbit's tail. Unmistakably a rabbit's tail.
"-doing?" Your girlfriend's mind seemed to have unfroze from her still state. You panic, struggling to decide if you should tell her or just let her find out for herself. You decide to hide it away from her and let her find the new addition to her body herself.
"Uh, n-nothing!"
You look at how her pants have moved down due to her tail getting in the way. And as soon as you noticed it, she noticed it. You swallow your pride and try to explain, only to be cut off.
"I'll never know how you talked me into getting these pants. There's no tail room in these things"
She looks back up with you, shocked at first and then a bit flirtatious. "Did you buy me this so I could show off a bit more?". She wrapped her arms around you and shook her tail around flirtatiously, jiggling it all about.
She kissed you and then moved back to the living room. "Shame about that remote not working. Do we still have the receipt? We could return it"
You, as shocked as you are, immediately bolted an answer. "No, it's fine, I might... tinker with it, found out how it works. ...o-or isn't".
She smiles. "Alright then. Gives you a nice project to work on".
You awkwardly laugh and agree as she walks into another room, and as she walks away, you couldn't help but stare at her tail. You couldn't believe it. The remote gave her a tail. An actual tail. And she believes that she always had it! This is insane. Crazy. ...but this could be fun.