Jennifer Lopez's Costar's Ex Claims She Faked Relationship to ...

Jennifer Lopez had her former costar Ryan Guzman fake his relationship status to hype up their movie almost 10 years ago -- so claims the guy's ex-girlfriend, who was with him then.

Melanie Iglesias called out J Lo on the "No Chaser" podcast for how she claims Jen acted during the 2015 "The Boy Next Door" press run ... alleging she would hold hands with Ryan at after-parties and act like they were an item -- knowing fully well he had a GF.

The Maxim model says she even met Jennifer at an awards show while she was dating Ryan ... claiming she gave her an awkwardly firm handshake, and an intimidating once-over -- something Melanie says she was disappointed over seeing how she viewed Jen as a hero.

Melanie also pointed out when Ellen DeGeneres directly asked on her show, "Doesn't he have a girlfriend?" ... J Lo played dumb ... claiming she didn't know. That's not exactly what Jennifer said, BTW -- but yes, she definitely made it seem like she wasn't sure.

Anyway, Melanie goes on to hint what she shared is just the tip of the iceberg .. and the full story is far worse. She doesn't get into details -- but it sounds messy, to say the least.

On its face, it seems J Lo's approach -- as described by Melanie here -- worked, as media reports at the time fueled rumors the two were a couple. Melanie, however, is now saying the whole thing was calculated on Jen's end ... and she has hard feelings over it all.

As she explains ... she was dating Ryan at the time, and says Jen knew that full well.

As for Melanie and Ryan ... they ended up breaking up the same year, although it’s not clear if J Lo's alleged actions were the direct cause of their split.

Mel certainly seems to think so, though ... that much we can hear. She also takes this shot at Jen -- "Why do you have to act like you're with everyone you're in a movie with?"

We've reached out to Jen for comment ... so far, no word back.