Taylor Lianne Chandler, who dated the 29-year-old US swimmer for months, wrote a long post on her Facebook page on Nov 14 in which she revealed that she was "born intersex and named David Roy Fitch" at birth.
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She also revealed that her life had been dogged by tragedy, admitting she left home at 15 and was brutally raped a few years later.
She said: "I originally changed my name to Paige Victoria Whitney.
"After a brutal rape and a horrible trial in 1993 I went away to get my surgery and changed my name to Taylor Lianne Whitney to escape back into anonymity and have a normal life."
"I was born with male genitalia with no testicles, but I also have a uterus and no ovaries," the 41-year-old later told Radar Online in an interview.
Taylor met Olympic swimmer Michael Phelps on dating app Tinder, and was with the Olympian when he was arrested for drink driving in September.
He has been suspended from competition and checked himself into a rehabilitation facility in October.
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While Taylor admits that she never told the athlete about her complicated past, but has emailed him about it after his arrest.
She said: "I never lied to him.
"We were together for such a short period of time, I never had a chance to tell him about my life."
She also told Radar Online that the intimacy she felt with Phelps was "amazing" and it was the first time "someone has made me feel like a real woman".
However Taylor added that it remains to be seen as to whether or not they will still be together after his rehabilitation.
"Michael is a brand, and dating someone like me may not be the best thing," she told Radar Online. "I just hope he follows his heart and not what his people tell him."
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Since the press have picked up on her story, often using the headline 'Michael Phelps' girlfriend was born a man', Taylor has lashed out on social media, objecting to being called "a man".
She wrote emotionally on Facebook: "Two steps forward and 100 steps backwards. That is what life feels like to me right now.
"In a world of educated people that had all the facts of intersex and what it means and then to sell a magazine say Michael Phelps is dating a MAN.
"I have cried now for an hour, thrown up anything in my system till I dry heaved. I just can't believe this is happening in 2014!"