Obviously there's some fluid trapped in there somewhere. It could be many different places. It could be subcutanious, could be intersticial, could be stomach, could be colon. Many different options.
If it's not life threatening don't worry about it. Keep cleaning and feeding and whatever it is it'll resolve as total health improves. She just won't be able to sneak up on you that's all.
Clean and feed, clean and feed, clean and feed. Gets old after a while, but it works.