I agree with Brent, more than likely you have paid off all of your debt without creating new debt (ex: credit card). Without anything to drive your scores up, they will drop. You've probably heard that no credit can be as bad as bad credit ... there's some truth to this. DC has a number of great first-time home buyer programs like HPAP, but in order to get real answers you should talk to a lender. I don't know Brent but his information is very insightful, maybe give him a call. If you need help with the ends-and-outs of being a first-time home buyer in Washington DC, i will be happy to assist you.
All the best!