Sources on the set of "The Hills" fear that Heidi Pratt (Montag) is developing a dangerous addiction to painkillers -- months after her recovery from plastic surgery.

"She's a shell of her former self," a show insider tells the new issue of Us Weekly, on stands now. (A rep for the reality star, 23, tells Us, "This is completely ridiculous.")

Indeed, Heidi and her husband, Spencer, have developed increasingly weird behavior. While their 2008 elopement to Mexico was done with a knowing wink, they're recently "crossed over to this bizarre place," the source says.

Case in point: Heidi only agreed to get 10 plastic surgery procedures in one day last November "because she knew it would be a huge story," an insider says. (Laments pal Jason Wahler to Us, "I knew her when she was normal.")

"They are addicted to the fame," the source says. "As 'The Hills' winds down and people start to care less, they are desperate to get back that high."

Pals say they've always had a thirst for fame -- especially Spencer, who declared shortly after "The Hills" premiered in 2006, "I'm going to date those girls and blow that show up." (Before pursuing Heidi, he also went after Audrina Patridge.)

Says a source who knew Heidi then, "She was spellbound. If it weren't for him, she'd be a mom in the suburbs."

To find out how Heidi and Spencer blew through hundreds of thousands of dollars in recent months, how Spencer has gotten violent and lost control with his family and how they're trying to compete against the Kardashians and Jersey Shore cast, pick up the new issue of Us Weekly, on stands now.

And check out their exclusive video of Heidi trying to eat a hamburger after her surgery here!

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