'Jersey Shore' Star Ronnie Ortiz-Magro's Black Eye Mystery Solved

Ronnie Ortiz-Magro suffered a nasty black eye after his baby mama allegedly hit him with a phone and punched him in the face, and this is a DIFFERENT incident from the one last week where cops came rushing to his home.

TMZ broke the story ... cops were called to the 'Jersey Shore' star's Vegas home Thursday night for a custody dispute over the couple's 6-month-old daughter. Law enforcement sources tell us during their visit, Ronnie told cops about Jen's history of violence toward him.

Now we find out ... this was just the latest incident of strife and violence. Back on October 5, Ronnie was allegedly brutalized by Jen ... he says she punched him and struck him in the head with a phone.

As we reported ... Ronnie posted a pic of his injuries on Instagram, but never disclosed what caused the shiner.

We're told officers in Vegas informed Ronnie he needed to file a police report in New Jersey -- where the alleged attack went down -- if he wanted Jen arrested. Sources close to Ronnie say he has not reported the incident ... at least not yet.