Barbie had just spotted John and was getting ready to make his life hell when her body informed her she needed to pee. Badly.
She wasn't sure what to do. If she left to use a restroom she could lose track of John, but if she didn't get up and run to the restroom right now she would wet her pants. Her mind was starting to panic a little bit when she noticed a 'clothing' sub-section on the chronovac, and an idea popped in her head. Rather than lose track of John, she scanned herself in quickly and changed the settings on underwear to a diaper.
Her face turned a little red as she felt her panties thickening into a soft, warm disposable. Before she could even realize it, her bladder had given out and filled her new garment with warm pee.
Barbie decided not to think about that, and to focus on her chance at revenge.