If you did Clark's flush and did not drink water, this often has happened over the years you probable were dehydrated.
Next time drink water in the afternoon according to Andreas Moritz , just keep drinking 1 hour away from the concoctions.

Could also be stones being reabsorbed into your system, stones that you did not pass all the way, helps to do the enemas. It's different for everyone, I get the nausea as well, but about 4 hours after taking the lemon/oil, then it passes within about an hour. Not a doctor, not a specialist, in fact I'll probably be having my GB out soon, just thought I'd suggest a few possibilities.

Enemas, enemas, enemas. The symptoms occur when the stones and parasites sit in your colon for too long. I suffered like you on my first flush because I didn't know about the enema solution. The past three flushes, I've been able to ease the symptoms with water enemas and, if the headaches and weakness are particularly severe, coffee enemas.