Amanda Bynes acted so bizarre during a workout class in Hollywood Tuesday night ... her instructor stopped the session halfway through and kicked her out -- TMZ has learned.
Gym sources tell us, Amanda was attending a 50-minute spin class at Equinox when she suddenly stopped participating in the class and aimlessly walked around looking to switch bikes.
Once Amanda found a replacement -- closer to the room's giant mirror -- we're told Amanda started cycling again, but removed her top, revealing a "tiny black strapless push up bra ... not a sports bra."
Roughly 25 minutes into the class, we're told Amanda stopped cycling again -- this time to pick up her Louis Vuitton purse ... and reapply her makeup.
According to sources, the actress was doing her makeup -- lipstick, eyeliner, the whole shebang -- for ten full minutes before the instructor grew furious and told her to get out, claiming he couldn't BELIEVE someone was doing makeup in the middle of his spin class.
Amanda then left the room and the class continued. For the record, the actress has not been permanently banned from the gym.