Ja'Net was suffering, silently apparently, from peripheral vascular disease and chronic kidney disease for years, according to the document. It also says hypertension was a contributing factor in her death.

TMZ broke the story ... Ja'Net died unexpectedly in her sleep last month. She was found unresponsive at her daughter's home in Glendale, CA. According to the death certificate, she was cremated. Some of her ashes were scattered at sea and the family is keeping the rest.

As we first told you, Ja'Net was in good spirits and seemed healthy during a "Good Times" meet-and-greet in early February, where she looked happy next to costars Johnny Brown, Ralph Carter, Bern Nadette Stanis and Jimmie Walker. Looks can be deceiving.

Ja'Net's family also said she had no known ailments or illnesses before her passing -- but the coroner's autopsy clearly showed otherwise.

Ja'Net was famous for playing the Evans family's sassy neighbor Willona Woods on the classic '70s TV series. She also composed and sang the theme song, "Movin' On Up," for "The Jeffersons."