We are learning more about the circumstances surrounding the death of rapper Lil Poppa ... and it's pretty shocking.

TMZ broke the news ... the rapper -- real name Janarious Mykel Wheeler -- was pronounced dead yesterday in Georgia at the age of 25.

Hapeville Chief of Police Bruce Hedley tells TMZ ... Lil Poppa was in a single-car accident on Interstate 85 south of Hapeville ... which is when he called his manager asking what he should do. The car was still drivable.

The manager told him they'd meet up in a nearby parking lot at the Hilton Hotel ... and Hedley tells us when they met up, Lil Poppa stayed inside his car as she spoke to his manager through the rolled-down window.

We're told it's unclear what they discussed -- but at some point, the rapper pulled a handgun, put it to his head and pulled the trigger, shooting himself dead. Hedley tells us ... an off-duty officer who was working at the hotel called 911.

Lil Poppa was rushed to Grady Hospital, where he was pronounced dead ... no suicide note left.

Lil Poppa was signed to Yo Gotti’s Collective Music Group (CMG) and has released several hit tracks over the last few years, including "Love & War," "Mind Over Matter," and "HAPPY TEARS."

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