'Clerks' star Lisa Spoonauer had been sick for years prior to her death last week ... still her family says her death was far from expected.

Sources close to Lisa's immediate family tell TMZ ... she'd been battling several chronic illnesses for 10 years prior to her death Saturday. The illnesses were not disclosed, but were described as degenerative.

We're told Lisa's family was aware of her ailments, but never suspected her condition was bad enough to kill her. Although she'd been in and out of hospitals for years, she died at home.

The coroner says Lisa died of natural causes, which her doctor signed off on -- a sure sign one of her many illnesses triggered the death.

"Clerks" writer and director, Kevin Smith, shared memories of casting her in the movie that changed his life. Lisa was 44.

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