The MTV star's sister broke the news on Facebook Sunday -- penning a heartfelt message to her late sibling ... which has since been deleted. However, a source with direct knowledge has given us a bit more insight into what happened.

We're told Autumn passed away Saturday at a home she was living at with her kids, her mom and her stepdad in Henrico County, Virginia. Our sources say her mother and stepfather found her unresponsive in her bedroom, after a friend notified them that something was wrong. Paramedics then arrived and attempted to render CPR.

Unfortunately, those life-saving efforts were unsuccessful -- and in terms of what may have caused this, we're told the family still doesn't know for sure. However, we are told that Autumn had been sick and was complaining of heartburn of late and expressed interest in seeing a doctor.

Our sources say she was also diabetic and was having kidney issues as well ... not to mention problems with her blood pressure. Bottom line ... it's unclear what caused her sudden passing. The family is working with law enforcement on the death investigation.

Henrico County PD are currently classifying the incident as a death investigation and are working with the medical examiner to determine the exact cause and manner.

Autumn was best known for appearing on season 5 of "16 & Pregnant," which documented her giving birth to her eldest son, Drake, who she welcomed with then-boyfriend Dustin Franklin. Autumn had three children in total.

There's been another recent "16 & Pregnant" tragedy -- we broke the story ... Sean Garinger was killed in an ATV accident, and he too left behind little ones.

Autumn was only 27.