The 24-year-old actress - whose samples came back positive after a random testing - tweeted her confession on Saturday (18.09.10) before attending a late night dinner with friends.
While at the Magnolia restaurant in Hollywood Lindsay was apparently spotted sipping alcoholic drinks, a violation of her probation order.
An onlooker said: "She had little nibbles of food and seemed to enjoy her night out. Lindsay`s friends ordered vodka-based cocktails, and Lindsay initially kept sneaking sips from their drinks.
"She was aware that other dinner guests were watching her, and it seemed silly that she thought she was being sneaky and that no one would notice that she was drinking her friends` cocktails."
Lindsay - who served just 13 days of a 90-day prison sentence in July after violating the conditions of her probation for a 2007 driving under influence (DUI) conviction - did not appear worried about the possibility of being sent back to jail for violating the terms of her probation.
The onlooker added to People.com: "She looked very happy and relaxed. The news about her flunked drugtest definitely didn`t seem to have dampened her mood. Lindsay was smiling, chatting with her friends and texting nonstop."
Los Angeles Superior Court Judge Elden S. Fox had previously warned the `Machete` star that if she committed another probation violation she could be put back in jail.
Meanwhile, it has been claimed Lindsay is broke and her mother Dina is worried about how she will pay her daughter`s lawyers` fees.
A source close to the actress told RadarOnline.com: "Lindsay is in serious trouble financially. Dina is struggling to make ends meet and pay Lindsay`s lawyers and this latest development doesn`t help things. Everyone thinks the Lohan family is so well-off, but if they knew the truth, it would be shocking.
"She`s in a tight spot, the family is in a tight spot."