Is someone suddenly shy about showing off her famous booty?
This reality star has no qualms about posting barely-there bikini pics on Twitter but didn't seem to want her curvy derriere photographed after a workout yesterday.
Was she wearing see-through Spandex or something?
Find out who's hiding her rear after the bump jump...
It's Kim Kardashian!
The Keeping Up With the Kardashians star and Quick Trim spokeswoman has been hard at work on her fitness—already hitting the gym twice today.
"Great workout this morning! What about u guys? I feel so lazy if I don't get a workout in," she first posted on her Twitter.
A few hours later, she added: "Just finished another workout...needed some extra cardio. Now off to work! Hope ya'll have a productive day!"
Despite Kim's extra sweat sessions, she wants you to know she's not in a weight war with her sister Kourtney for a better bod. OK?!
See more stars working it in our Sweatin' With the Stars gallery!