Lindsay Lohan switched rehab centers from the Betty Ford Clinic to Cliffside Malibu-and now we know why. Apparently a contraband substance was brought in and there was an underground drug outbreak.

Allegedly, the visitor of a fellow patient bought a prohibited substance in-probably Oxycontin- and several other girls consumed it. Lindsay insisted she didn't, though....and she was upset enough to leave Betty Ford to check into Cliffside Malibu.

"She heard what was going on and was really upset by it and wanted to get out," the source said. "She's been doing really well detoxing and didn't want to be around the temptation of more drugs... Lindsay also wanted to be at a place where she would receive more one-on-one treatment time."

Apparently visitors to rehab centers like Betty Ford aren't thoroughly checked for drugs. Although patients' belongings are searched when they enter rehab, visitors have it fairly easy, just checking their phones. Visitors are not searched for banned substances.

The mom of a patient who was at Betty Ford with Lindsay said that she had to transfer her daughter also. There was a scandal within Betty Ford after several women were given the drug.

"She was afraid of a relapse," the upset mother said.. "Some of those who took it have now left voluntarily."

There have been murmurings of Lindsay wanting to switch rehab centers for a while. A few weeks ago, Lindsay's dad, Michael, visited her and said he wanted her out. Although he was impressed with her progress and said Lindsay was being a "model patient", Michael still thought Betty Ford wasn't the right place for his daughter.

Betty Ford is a 30-day program, and it "starts getting repetitive," a source said. Because Lindsay is court-ordered to rehab for 90 days, she wanted somewhere with more individual therapy that was less repetitive.

Still, one source said that Lindsay wasn't an angel, even if she didn't partake in the smuggled Oxycontin. The source said that having Lohan there was "problematic" because she "couldn't follow the rules". But her dad said she had made a total turn-around.

Hmmm. Let's hope LiLo can follow the rules at Cliffside in Malibu.