Corinne Olympios was popping pills and mixing booze during "Bachelor in Paradise" filming before the infamous swimming pool hookup with DeMario Jackson ... TMZ has learned.

Sources close to production tell TMZ ... Corinne made the admission during Saturday's taping of the 'BiP' reunion. We're told she didn't elaborate on what kind of Rx, but she's adamant -- ingesting the pills that day and mixing it with booze intensified her drunkenness, which she says led to her blacking out. 

TMZ broke the story ... production shut down following allegations of sexual misconduct in a pool. A producer felt Corinne was incapable of giving consent, but the video and interviews with various members of cast and crew led Warner Bros. honchos to conclude ... she knew what she was doing.

It's interesting ... when 'BiP' filming resumed, new policies were implemented, such as checking bags for prescription drugs, legal or otherwise. Now it makes sense.

Obvious question -- why'd she mix meds with booze? We're told Corinne pled ignorance.