Annamarie:
There is no reason to worry. Indeed, you should consider yourself very lucky, and really look forward to the opportunity.
At NIH they will question you, take blood and other samples from you, put various devices on you, take various measurements, and so forth. You'll feel a little like a guinea pig.
On the other hand, this may be the first time that a whole medical team that knows a lot about SCLS will take your problems seriously, make a big-picture evaluation of your case, and give you and your own doctor the best advice that is available. And they do that at no cost to you!
Thus, you will feel very grateful for the experience, and you will join a distinguished line of SCLS patients who have inconvenienced themselves a little bit in order to learn more about their medical problem -- and to advance medical knowledge of use to all of us SCLS patients from around the world.
Arturo
SCLS NIH Patient Volunteer #1
PS: You can also do some sightseeing while in Washington.