I cannot really add anything useful, but my progression was so similar in all senses that I thought I should comment: it started after an intestinal infection I got in India, many different hypotheses until they settled on Reiter's syndrome/reactive arthritis. Then for 12 years I was without real symptoms or medication, though with almost chronic back pain. A new bout 12 years later (knees like grapefruits etc), diagnosed now as ankylosing spondylitis. In fact I had understood that this was a specific form of reactive arthritis (rather than something different), but probably you're right.