I can't catch the meaning of this sentence: 'I had a black plunge of shame.'
Here is the passage from "The Magus" by John Fowles:
Someone had knocked on the door. I was staring at a wall. I was in bed, I was wearing pajamas, my clothes were folded on the chair. It was daylight, very early, the first thin sunlight on the tops of the pines outside. I looked at my watch. Just before six o'clock. I sat on the edge of the bed. I had a black plunge of shame, of humiliation; of having been naked in front of Conchis, of having been in his power; even worse, others could have seen. Lily. I saw myself lying there and all of them sitting and grinning while Conchis asked me questions and I gave naked answers. But Lily — he must also hypnotize her; this was why she could not lie.
Google doesn't help.Thanks for help.