Don't mess with Penny.

That's Blake Lively's response to photos that surfaced in June of the Gossip Girl star walking outside with an obvious stain on her dress.

The culprit rumored to be responsible for the mark? Lively's adorable three-year-old malti-poo, Penny, whom she carries everywhere.

In fact, Blake Lively blames herself.

"I spilled something on my dress. I didn't want it to stain, so I stuck my dress under the sink and was running out the door," Blake, 20, tells USA Today. "I'm holding Penny, so every magazine prints that Penny peed on me. It wasn't Penny!"

Ensconced in a Midtown hotel suite with Penny sitting next to her on a plush pink dog bed, Lively leans over and pets the pooch.

"She eats bones and stuff. She's a real dog," Blake Lively says. "I bring her with me everywhere. It's nice to have another heartbeat. She's a little piece of home."

And she's a little bit of comfort in Lively's increasingly hectic life.

The actress is most famous for playing posh prep-school party girl Serena van der Woodsen on Gossip Girl, which returns September 1.

And this week, the blonde beauty reprises her turn as plucky Bridget in The Sisterhood of the Traveling Pants 2, the sequel to the 2005 female-bonding flick that marked Lively's major movie debut.

This time, the four friends (Blake Lively, America Ferrera, Alexis Bledel and Amber Tamblyn) are college students grappling with major changes in their lives, with Bridget heading to Turkey for an archaeological dig.

"Blake is a young actor but very thorough. I can see why she's on the rise," says Sisterhood 2 director Sanaa Hamri. "Blake was such a big advocate of the Sisterhood. She loved Bridget. When she feels something is right, she follows her heart, no matter what. She's fearless and down-to-earth."

Like her character, who "goes across the world, I was moving from Los Angeles to New York literally at the same time I was shooting this movie," Lively says. "I was moving away and starting out on my own. And I have these girls — people I work with, people on the show — that are really close."

As for the persistent rumors that Lively and Ferrera, now the award-winning star of ABC's Ugly Betty, were less than eager to do a sequel?

"I never read those stories. The only thing that made any of us nervous was the fact that we had a six-week prep," Blake Lively says.

"It happened so fast. We were nervous because we didn't have a script at the beginning. We were all so happy to be together. We loved the first movie so much. We wanted to do a sequel. There was never any hesitation about that."

There is, however, major hesitation on Lively's part when it comes to discussing her Gossip Girl co-star and off-screen beau Penn Badgley.

When told they make a cute couple, Lively responds with a stilted "Thanks... Yeah, I don't really like to talk about anything but the show. It's two separate things. You don't date a character. I'm nothing like Serena."

In fact, Serena's life goes through major changes on Gossip Girl's Season 2 after she and Dan (Badgley) ended their relationship.

"She has had the whole summer to be away from Dan," Lively says. "She's able to come into her own. She becomes a pretty important New York socialite outside high school. And Dan — there's still some residual chemistry."

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.