Hi Cathryn,

I remember when I lived in San Diego, I used to get those feelings quite often. When I was in the navy out there, we would go out to sea and the feelings would persist for days when I got back to land. We called it "sea legs". The funny thing is, once in a while I'd have that sensation when I hadn't been out to sea in a long time, which made me feel like the land mass was actually shifting. What your feeling is probably real, there could be a few millimeters or more of lift and sag which would be undetectable by GPS systems, but discernible by the human body attempting to reach equilibrium.

All the best, Billion