Some of AA's fare classes consider DCA and BWI (and IAD) to be co-terminals in the same city; but some consider only DCA and IAD to be the same city, with BWI as a different city. The EXP desk recently made us cough up $200 to change an itinerary for me and Mrs. Bobo from BWI to DCA because it was a "different city."

Unfortunately there seems to be no easy way to check the conditions of the fare for a particular PNR on aa.com. Either way, you should be entitled to change or cancel your reservation without charge if they can't get you to the same airport that you were ticketed on.