Go for the cutesy answer to hold character and keep things moving along. Something along the lines of "You were so pretty today I couldn't think of anything but (insert Her favorite bedroom play)". Keeps the mood, answers the question and provides a somewhat valid reason for a playfully kinky "punishment" if desired.

Your girlfriend is very cute. :)

you should have "punished" her

Maybe she's just really good at acting her part?

Aww, that's so adorable. She's sounds so innocent and oblivious. Maybe she's never done this before. She could have had a rough childhood and possibly abused and this was a trigger, though it seems more likely that she'd cower and plead you stop if that were the case.

really?

At least she didn't break down and admit to cheating on you lol

or maybe she thought she'd been caught :-P

She got into that fairly quickly. ;)

I think that's kinda cute