Miley Cyrus shows off flat stomach during Costa Rica vacation

Rather than enjoying rest and relaxation while on vacation in Costa Rica, Miley Cyrus has been focusing on her fitness -- and she's showing off the results. The singer, 20, posed for a photo that showed off her skinny frame and flat stomach in a sexy outfit.

PHOTOS: Miley Cyrus' skimpiest outfits

Rad and Refined shared the shot on Facebook on Sunday of Cyrus blowing kisses to the camera while posing in their colorful cropped tube top and cut-off denim shorts. While vacationing with her fiancé, Liam Hemsworth, and his brothers Chris and Luke, Cyrus has been photographed flaunting her toned body while doing yoga on the beach.

In addition to staying fit, the former "Hannah Montana" star has been tweeting about trying to stick to her diet. "Too. Much. Pizza. Back on the health train tomorrow. Cheat night," she wrote Jan. 24. "You can't always control what goes on outside. But you can always control what goes on inside."

PHOTOS: Miley Cyrus and Liam Hemsworth's love story

"I now remember why I never eat pizza. It makes me feel like s---. Glutenized," she wrote, but then quickly added, "Let me clarify 's---' wasn't meaning 'fat.' I mean sick. Because gluten is poisoning me. Your stomach has a brain too and it hates s--- food."

"Ate too much pizza and French fries," Cyrus explained. "Yoga and a swim in the a.m."

PHOTOS: Miley's hair evolution

And it seems Cyrus plans to go on a raw diet -- but she treated herself to one more cheat day first. "Back to raw tomorrow," she tweeted Jan. 27. "Right after these birthday cake Oreos."

MORE ON WONDERWALL:

Celebs Without Makeup

Best and Worst Beach Bods

Most Famous Celeb Pets