TMZ broke the story ... the woman told cops Cee Lo took her to a downtown L.A. restaurant in July, where she claims he slipped a drug in her drink. She says the next thing she knew she was laying naked in a bed with him in a room. Cops are investigating the allegation and we're told the case will be referred to the L.A. County DA.

Sources connected with the case tell us ... the woman and Cee Lo met at the Super Bowl in Dallas in February, 2011. They hung out over the weekend. She went back to Houston, where she lives, and Cee Lo returned to L.A., but they stayed in touch and dated off and on for the next year-and-a-half. In addition to seeing each other (he flew her out to L.A. from time to time), they frequently texted and spoke on the phone.

Sources tells TMZ ... the woman sometimes asked Cee Lo to buy her expensive things and even asked his friend how she could get him to buy her expensive things.

Sources connected with Cee Lo tell us ... on the day in question, he had flown her out to L.A. They went to the restaurant and afterward both took Ecstasy and stayed up all night. One source says, "No one was unconscious."

Our Cee Lo sources claim the woman fabricated the story because she's involved with someone back in Texas who got wind she was in L.A. with Cee Lo and she was just covering her butt.