'Jersey Shore' Star Ronnie Unsure Why Ex Came Over to Allegedly Attack

Ronnie Ortiz-Magro doesn't know why Jen Harley was allegedly snooping around his house before launching an attack with an eyeliner ... they're not dating and there was no invite.

Sources close to Ronnie tell TMZ ... they are absolutely not back together and Ronnie did not tell Jen she could swing by his place the night of the alleged attack.

We're told Ronnie finds it puzzling that Jen would show up on his doorstep ... after all, she's got a restraining order against the "Jersey Shore" star.

As for how Jen could've possibly slipped inside ... law enforcement sources believe she still had a key to his place. We're told there were no signs of forced entry.

Step one for any breakup ... change those locks!!! Can't say Gloria Gaynor didn't warn ya.

TMZ broke the story ... Ronnie filed for legal protection against Jen after the alleged attack, claiming she was enraged after finding another woman's makeup in the trash.

We asked Jen's attorney, Lisa Bloom, why Jen was in Ronnie's home and she says ... "You ask a good question, and there is a good answer, but we will be presenting her position in court, not in the media."