HOLLYWOOD, Calif. — Demi Lovato's decision to enter rehab was fueled by her heavy drinking and cocaine use, an explosive new report alleges.

College student Brian Payne claims he witnessed Lovato's drug use firsthand when they both attended the same Texas house party last December.

"She was doing line after line like a pro — and she was 17 at the time," Payne charges in an interview with Life & Style. "I just remember her doing it as if she had been doing it for a long time. It didn't seem like something new to her."

Lovato, 18, quit her Camp Rock tour with the Jonas Brothers two weeks ago. She reportedly sought treatment after getting into a fight with a female dancer on the tour.

So what caused her downward spiral? A source tells the mag that being on tour with her ex, Joe Jonas, certainly didn't help matters.

"Demi's been on a tear lately," explained the insider. "She'll chug booze straight from the bottle."

The young starlet's reps have been vague about the issues at hand, only saying that she's currently being treated for "physical and emotional" issues. As for the rumored drug problems, a source denied the allegations to People, but did admit that Lovato's long struggled with eating disorders and cutting.

"Demi is not in treatment for drugs," the insider insisted. "She is there to deal with emotional and physical issues she's been battling her whole life,"

Also see: Demi Lovato Pictures