Whoa! Ashley Tisdale Shows Off Butt Tan Lines in Skimpy Bikini

Credit: Instagram.com

While her BFF Vanessa Hudgens is busy filming Spring Breakers down in Miami, Ashley Tisdale is having some fun in the sun on her own.

In a pic posted to her Instagram Thursday, the Hellcats star showed off her trim body and toned tush (with tan lines) in a skimpy brightly colored string bikini.

"What a beautiful day," the 26-year-old wrote along with the photo.

In February, Tisdale made her red carpet debut with her beau of two months, Boys Like Girls lead singer Martin Johnson.

The pair met back in 2009 when Tisdale starred in Boys Like Girls' "Love Drunk" music video; she was dating music video director Scott Speer at the time.

"They're still going strong," a source tells Us Weekly of Tisdale and Johnson. "She's a firecracker and Martin loves that."