I agree with urzu that the potato and butter probably came too soon and your body was not ready for something this heavy. But i bet it was soooo yummy ;).
After 18 days of water, optimally you would have about 4.5 days of juice only, then 4.5 days of fruits and non potato vegetables only before advancing to potatos and everything else. I think a month would be a lot longer than you would have to wait.