The Sexual Demon

I'm looking for a movie that I saw once - I don't have a title but can remember some things that happen in the movie.

A middle age women opens her door to give an attractive-looking gardener a drink of water. She invites him in. The story finds them in bed having
sex where he turns into the devil or something and rocks her boat.

She wakes up very scared and looking around her house, but all she seems to see all through her house is a black snake.

Anyone?