The Georgia Bureau of Investigation tells TMZ ... 40-year-old Chaka Stewart was found unresponsive in his cell Saturday at the Clayton County Jail in Georgia. We're told a guard and a medic who were making the evening rounds noticed Stewart lying there, and they attempted life-saving measures when they realized something was seriously wrong.

We're told he was pronounced dead at the jail. Our law enforcement sources say Stewart was alone in his cell, and no foul play is suspected. An autopsy is underway to figure out what happened.

Stewart's mother, Jacqueline Gordon, confirmed her son is the man who was arrested outside Rick's Georgia estate last week -- where cops say he crashed his truck and was carrying a firearm ... only to finally be apprehended at gunpoint following what was described as a crazy pursuit through the streets.

She says she last spoke to her son Friday, and claims he told her he was being mistreated behind bars. Gordon alleges Stewart said he was being beaten -- unclear if he was blaming officials or other folks behind bars -- wasn't allowed to shower and was confined to his cell for 23 hours a day.

Gordon also claims one of the deputies told Stewart he'd never be getting out of there ... while also asserting Georgia authorities have been ducking her and her inquiries.