Palm Brach County Sheriff's Office

POORSTACY 's baby mama has been arrested ... with cops alleging drug use in the presence of the couple's young child in the days before the rapper's death ... and we're learning she and the young child were in the room when the musician killed himself.

Nicole Grikstas was arrested Saturday hours after POORSTACY -- real name Carlito Milfort -- took his own life ... with cops booking her on a child neglect without causing great bodily harm charge. TMZ.com

According to a Boca Raton Police affidavit -- which cites an interview conducted with Nicole at the police station hours after the rapper was found dead in a hotel room -- cops say Nicole broke down the events leading up to his death by handgun.

Nicole told police the couple and their child che

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