Earthquake, that is.
I have felt several earthquakes during the time I've been in the Bay Area (and one before that, another story) but feeling 3 quite distinctly within 4 days is a new experience -- and as far as I can see, all about the same size and centered on the same location (
link likely to become stale, but shows the three in question at the top of the list right now).
(Updating just to add another
link that is probably less interesting but more likely to last :)