Chrisean Rock got thrown into handcuffs and hauled away by cops after she walked into a courthouse this week with her baby -- and now we have a sense of what it's all about. Sources with direct knowledge tell TMZ ..

. the rapper was taken into custody Monday after she entered a courthouse in the San Fernando Valley ..

. where we're told Chrisean was swiftly pounced on by officers. BTW, Blueface 's other baby mama, Jaidyn Alexis , was there too .

.. although it doesn't seem she was rolling with Chrisean.

Anyway, we're told 5 LAPD cops were already waiting for Chrisean when she walked in -- so it appears this might've been planned ahead of time ...

and as soon as she entered, they were on her. Our sources say 2 female officers searched Chrisean once they had her in bracelets -- and we're told her baby was taken away by officers as well. Once the full pat-down was complete, our sources say Chrisean was escorted out of the building .

.. unclear what happened to the baby.

Law enforcement sources tell us Chrisean was arrested on a felony fugitive warrant -- and that she's being held without bail. We're told there's an additional assault with a deadly weapon charge being tacked on here too, but we don't know if that's stemming from CA or elsewhere. In any case .

.. it appears the fugitive hold stems from an open warrant out of Oklahoma .

.. where she's wanted in connection to drug charges -- including possession of marijuana with intent to distribute, and possession of a controlled danger.