Lindsay Lohan may not have been acting like herself in the hours before her arrest ... 'cause she was fighting off a bad case of BIEBER FEVER.
Lohan attended the Justin Bieber concert at Madison Square Garden in NYC last night -- we're told she had pretty great seats, too.
It's unclear if Lohan digs Bieber's music -- or just thought the concert would be a good place to pick up chicks.
Of course, things turned pretty sour for Lohan right after the show ... when she rolled over to a Manhattan nightclub and allegedly punched a woman in the face.
She was arrested ... click here for the details ... and was eventually released.