What did you first think it was? My first thought was a major tractor trailer truck going down our street (we live in a suburb, would be very rare), but as the shaking got worse, I thought it might be Quantico having some sort of exercise (bomb-related), and at its worst point I thought it was some sort of attack (biggest fear).
Lesson learned - I thought I was calm in crisis...and I am so not!
Scariest part - couldn't get a hold of anyone, cell phone and house phone didn't work. Lines were totally jammed.
And FYI - if you're right around the main area (DC area), news is saying we may feel aftershocks or have another small earthquake anytime within the next few hours or days, just so you're aware!