How long had you lived in the DC area before moving in? It's pretty common for people to develop allergies, seemingly out of the blue, a few years after moving to DC.

Talk with your allergist and have the blood work done to see what you may be allergic to. You should have your ducts cleaned every few years, especially if you recently moved in as you have no knowledge of the home's true history and what may be lurking within. A client of mine had their ducts cleaned and the contractor found a video camera in the bathroom duct. They could also have decades of dust/smoke/pet dander or mold. Have them cleaned but be cautious as many companies will try to charge you a significant amount for a mold treatment in the ducts.