Rich Homie Quan -- Smoking Video Triggers Child Protective Services Investigation

Rich Homie Quan learned not to wave a red flag in front of a bull, because he just got gored ... Child Protective Services is now on his case for smoking what looks like a blunt in front of his kid.

Law enforcement sources tell TMZ ... Georgia authorities saw the video of RHQ's 1-year-old on his lap as he puffed away, and they were alarmed. Child Protective Services and the Fulton County Police Department have now teamed up to investigate.

Our sources say CPS wants to sit down with Quan and ask a slew of questions ... notably, what was he smoking? They also want to know if this was a one-off or his standard operating procedure.

Law enforcement tells us the probe goes beyond RHQ's fitness as a parent ... he could face possible criminal charges, including endangering a child and reckless behavior.