Hilary,
It can be very frustrating trying to find the cause of our pain. Do not give up or get depressed. Sometimes it requires us searching and seeing many doctors. I saw doctors into the double digits until I found one I was really comfortable with and allowed him to do the surgery.
My RA doc just assigned the reading on AS to me as well. I am much older than you, but have spinal problems throughout my entire spine. I also have a grandfather that I think suffered with this disease.
Dermatomes of the spine....
Hope this helps you to understand better....