Alexa Curtin from 'RHOC' ... I Was Raped by an O.C. Cop

6:25 PM PT -- The O.C. Sheriff's Department tells us the officer Alexa names in her lawsuit, Deputy Epson ... is not one of their employees. Nor has he ever been. However, a Sheriff rep tells us they take the allegations seriously and they are investigating.

The daughter of 'Real Housewives' star Lynne Curtin says an O.C. Sheriff's deputy raped her after a bizarre traffic stop and search of her vehicle ... and now she wants the county to pay.

Alexa Curtin says she was leaving her boyfriend's Dana Point home in April 2014 when she got pulled over by a man she believes to be an O.C. deputy. In a lawsuit, obtained by TMZ, Curtin says the deputy searched her car and found underwear. She says he grilled her, saying things like ... how many pairs of underwear she had and why she needed them.

Alexa, who used to make frequent appearances on 'Real Housewives of Orange County,' never says why he pulled her over in the first place.

After the search, she says the deputy abruptly announced he had to tend to another call, but told her to stay put on the side of the road -- only to return 20 minutes later in his personal vehicle, and in plain clothes.

Curtin says he jumped in the passenger seat of her car and said, "Since you are still here, I am going to f**k the s**t out of you." She says he groped her, digitally penetrated her and then told her to straddle him. She describes the encounter as "nonconsensual sexual intercourse."

She says he ejaculated inside her and on her seat.

In the suit, Curtin says she was afraid to report the alleged rape to authorities out of fear the deputy would harm her again. She's suing Orange County for violating her civil rights and for an illegal search and seizure.

The O.C. Sheriff's Department had no comment.