EXCLUSIVE

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.

We spoke with Vivid honcho Steve Hirsch who tells us Chandler's raking in 6-figures for "Going for the Gold" ... which features a Phelps look-alike and is set for release next week.

"This is the most unique footage Vivid has ever released," Hirsch says ... "The sky is the limit on this movie."

Go USA.