NBA's Jae Crowder -- Baby Mama Arrested ... Alleged Spit Attack at Celtics Game

Dramatic night for Boston Celtics forward Jae Crowder -- because his baby mama got arrested AT HIS GAME Monday night for allegedly spitting on another woman at Staples Center.

TMZ Sports has confirmed ... Dana Lambert was arrested for simple battery last night while Jae's Celtics were in the middle of playing the L.A. Clippers.

Multiple law enforcement sources tell us Lambert got into a heated argument with another woman at Staples ... and things allegedly came to a head when Lambert channeled her inner llama and spit on the other woman.

We're told the two women involved have a long-standing beef with each other.

The other woman called for security and demanded they make a "citizen's arrest" -- which ultimately happened. Lambert was eventually taken into custody by the LAPD and booked for misdemeanor battery.

She was released early Tuesday morning after posting $20,000 bail. Calls to Jae and Dana have not been returned.