The 'Mean Girls' actress completed 90 days of court-ordered treatment for substance abuse at the Cliffside Malibu medical centre in California earlier this month and a fellow patient alleges she spent countless hours browsing internet stores for clothes and spent up to $5,000 a day.
The source told Radar Online: "Lindsay would shop online all day. It was like an addiction. She would drop $5,000 a day no problem. Mostly on clothes."
The 27-year-old starlet allegedly had her extravagant purchases delivered to the rehab facility until staff warned her to curb her spending.
The insider claims: "The patients and the staff were so annoyed by all of the boxes being delivered. Eventually the clinic ran out of room to house all her purchases and told her she couldn't order any other items. It was crazy."
Lindsay "bragged" about having access to the credit card of a "rich guy in New York City" and used his bank account to treat herself.
The source said: "We never asked who he was or why he would do that, but she didn't seem to have any hesitation about using his money for whatever she wanted."