When I first started feeling the kittens move, I wasn't sure if it was that or if it was just gas. It started out feeling like a "bubble" is the best way I can describe it. Closer to delivery, it becomes VERY noticeable what you're feeling. You can sometimes feel individual body parts. It feels like they're rolling around or kicking.
The last time I had a pregnant kitty was when I was much younger so I can't quite remember. All I know is that my previous cat was constantly hiding. I guess you could feel a bump there. I definitely noticed a change in usual habits.