Michael Phelps‘ self-proclaimed ex-girlfriend just had a close encounter with about a billion little swimmers … after shooting her very first hardcore porno flick … TMZ Sports has learned.

The “star” is Taylor Chandler — who claims she had a relationship with Phelps after meeting him on Tinder. She also says she’s “intersex” … which means she was born with both a penis and a uterus (she’s since had the penis removed).

And now, we’ve learned Chandler is showing off what she’s got below the belt in a new X-rated flick she shot with Vivid Entertainment … the porn company famous for bringing Kim Kardashian‘s sex tape to the world.

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