Heavy D died from a pulmonary embolism caused by deep leg vein thrombosis ... this according to the Los Angeles County Coroner's Office.
A pulmonary embolism is a blockage of one or more arteries in the lungs -- most often caused by blood clots which travel in from other parts of he body. In Heavy's case, the clot was formed in his legs.
Officials say Heavy also suffered from heart disease, which could have been a contributing factor in his death.
The Coroner's Office has classified the death as "natural."