The 1980s icon opened up to People Mag ahead of her new Hulu project, 'Pretty Baby,' which drops next month. After having already teased the fact she was assaulted in her youth, she describes it here for the first time ... and says it went down during her college years.

Shields says she was a recent Princeton grad and went out to dinner with an unnamed Hollywood exec, whom she thought was going to land her a new acting gig ... this after her career had somewhat flatlined. Unclear where/when this happened -- she doesn't say.

Shields says he invited her back to his hotel afterward so she could make a call -- and she went with him. Once there, she claims he sexually assaulted her ... but doesn't go into much detail. All she says is she didn't fight back -- and felt guilty afterward.

Shields says of her thoughts at the time ... "I kept saying, 'I shouldn't have done that. Why did I go up with him? I shouldn't have had that drink at dinner."

She went on to describe her reaction like this ... "It was really easy to disassociate because by then it was old hat," she recalls. "And because it was a fight-or-flight type of choice. Fight was not an option, so you just leave your body. 'You're not there. It didn't happen."

We got Brooke earlier this week on camera, and she talked to us about the new doc as well ... getting emotional about it, and telling us just how hard her life had been.