Paul Michael tells TMZ ... two weeks before Amanda's breakdown, she had disappeared for 3 days and finally came home with another guy. They argued, she said she didn't care about him anymore and they ended the relationship. Paul packed up his things and left their home.

Paul says he was shocked at her disappearance and behavior, because up to that point, she seemed stable ... focusing on her nail care business and seemingly happy.

He thinks something happened to her and whatever it was happened quickly, because when she returned home with the guy Amanda seemed like a different person. A little more than a week later, Amanda was in desperate shape.

TMZ broke the story ... Amanda was walking near Downtown L.A. March 19 without any clothes. She waved a car down and told the driver she was coming down from a psychotic episode. Amanda borrowed the driver's phone and called 911 herself. She was placed on a psychiatric hold for 3 and a half weeks.

Paul says he hasn't spoken with Amanda since he left their home.