Francesca Eastwood (L) and artist Tyler Shields attend Tyler Shields debut of MOUTHFUL presented by A/X Armani Exchange in support of LOVE IS LOUDER at a Private Studio on May 19, 2012 in Los Angeles, California. Credit: John Sciulli/Getty Images for LOVE IS LOUDER

Lindsay Lohan's bizarre Tuesday night freakout during Francesca Eastwood's birthday party left everyone scratching their heads.

"We were celebrating my girlfriend Francesca's birthday at Bootsy Bellows, and it was a blast," Eastwood's photographer beau Tyler Shields told E! News Friday. "Suddenly, there was a burst of chaos, and as quickly as it started it was over. After the fact, we were all a bit confused about what just happened."

The birthday girl agreed she and her pals were all baffled by the troubled starlet's behavior -- but added that it didn't put a damper on her celebration.

"I'm not sure what exactly sparked Lindsay's outburst," Eastwood said. "Regardless, I still had an amazing birthday and Parent Trap is still my favorite movie."

On Friday morning, a source told Us Weekly that the Liz & Dick actress, 26, appeared to be back to her old, headline-making antics at Bootsy Bellows, putting a wrench in E! star Eastwood's birthday bash.

"Francesca was celebrating with around a dozen friends in a private area when Lindsay came over and started screaming that Francesca should leave," the source says. "She was yelling, 'I'm a star, she's a nobody, get her out of here!'"

Continued the witness, "One poor guy came over and tried to calm [Lindsay] down and she acted aggressively. At that point security told her to leave and it was totally embarrassing. She is acting like some bad '80s film star, and it is hard to watch because she needs help."

But before Lohan embarked on her journey home, she and her party made one more cringe-worthy snafu.

Says an onlooker, "Lindsay's friend was driving and they sped out and almost hit one of the valet guys!"