Crap is about to hit the fan in CeeLo Green's felony drug case -- at least according to the L.A. County D.A., which just hinted it's on the brink of exposing the FULL story surrounding the singer's ecstasy charge, a story that involves allegations of rape.

CeeLo was back in court today -- when the judge set a date for a preliminary hearing in January -- and prosecutors said they expected the prelim hearing to last several hours.

The reason ... prosecutors said there's a lot more to the drug charge than meets the eye, insinuating they were preparing to delve into the nitty-gritty of the case ... i.e. the sexual assault aspect.

You'll recall, this whole thing started last year ... when a woman accused CeeLo of spiking her drink with ecstasy at a Downtown L.A. sushi joint. She claimed the next thing she remembered was waking up in a bed naked with CeeLo in the room.

CeeLo was eventually cleared of sexual assault due to insufficient evidence, but got hit with a felony drug charge instead.

But the funniest part of today's hearing -- CeeLo spent the entire time standing behind his lawyer Blair Berk, who was blocking him from the cameras. As we reported, Blair objected to cams in the courtroom because she felt they put CeeLo in the spotlight too much.

Overall, CeeLo didn't appear concerned today though -- he shrugged the whole thing off on his way out of court.