I feel drained the next day too, at least for the first 8 hours or so... last flush i had to get up and go to work and it was very undesirable. I don't feel like slaying any dragons until the 2nd morning after the olive oil, at which time i have been known to feel sensational :).
I would think that whether it is electrolyte related or not, the body is recovering from the difficult challenge of processing 7 ounces of olive oil and the liver's hard fought reaction to that.