Sources close to the situation tell us ... Monique Greene had a phone conversation with a friend last Thursday, and the friend hung up feeling Monique was severely distressed. We're told the friend called police and asked them to make a welfare check on Monique.

Emergency personnel responded and we're told they felt it was best to take Greene to a hospital. She agreed to go voluntarily. She was there for a few hours in the evening before being released. Our sources say there were no signs or talk of suicide.

Greene's attorney, Karen Koehler, tells us the friend who called cops was worried about Monique's "continuing severe emotional distress related to the alleged Nelly rape and the aftermath."

We broke the story ... Greene's suing the rapper for sexual assault and defamation, and she's also accused Nelly of similar sexual misconduct against 2 other women.