For the first time since her daughter's sudden death, Brittany Murphy's mother Sharon is speaking out, along with Murphy's husband Simon Monjack.

The pair told PEOPLE they believe the 32-year-old actress died of a congenital heart defect that was diagnosed in her teens -- not from a drug overdose.

"She had a little bit of a cold. You could never in a billion years imagine this happening," her mother said.

Of rumors that his wife suffered from a mental illness or an eating disorder, Monjack told PEOPLE, "Do you not think anybody would have picked up on this anorexic, mentally deranged person? It's ludicrous."