With the positive HLA-B27, the doctor is probably looking at one the spondyloarthropathies, a grouping that contains ankolysing spondylitis, psoriatic arthritis, reactive arthritis, and others. With AS there can be characteristic fusing in the spine and that might be the reason for that x-Ray. They are seronegative because there is not a positive RA factor. The positive ANA can lead your doctor to consider Lupus, Sjoogren’s syndrome, and others.
Sounds like your doctor is taking time to consider the many alternatives.