Amanda Bynes -- BOOTED from Gym Class for Weird Behavior

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.