[MissC]

Does this happen a few days out of every lunar cycle?

But seriously, I think AnnabelP is on the right track. My pendulum swings a bit too, but not to such extremes as I've always been pretty mellow and even-tempered.

Think of it like waves about a centerline. The higher the amplitude above the center, the lower it goes below -- the distance from center remains the same above and below.

Do you practice any form of meditation?