New details have emerged in the death of actress Michelle Trachtenberg ... her mother tragically discovered her daughter's body in bed Wednesday morning, just months after her liver transplant, TMZ has learned.

Law enforcement sources tell TMZ ... Lana entered Michelle's Manhattan apartment, only to find her unresponsive, lying on her back in bed -- Lana then called police and paramedics, who pronounced Michelle dead at the scene.

We’re told Lana last saw Michelle alive at 10 PM Tuesday, and she was clearly active despite her health concerns -- the "Gossip Girl" actress was even out with friends last Thursday night at Sartiano’s in NYC.

Our sources also confirmed that Michelle recently had a liver transplant, causing complications with her liver and thyroid. Earlier today, TMZ reported Michelle's body may have rejected the transplant, possibly leading to her death.

Despite her health issues, Michelle defended her gaunt and frail appearance online in recent months, insisting she was "happy and healthy."

Michelle's family has asked for privacy during this devastating time and didn't release further details of her shocking death at 39.