The tests that have been performed were done to find out the cause of sacroilitis. More frequently it is associated with other spondyloarthropathy, and the associations can be with inflammatory bowel disease, some remote infection like TB, psoriasis etc. If all tests are negative, it can be seronegative spondyloarthropathy, i.e. negative blood tests.
First line of treatment is NSAIDS - like voveran, that you are taking. This can be changed if its not effective.
Second line of treatment is sulfasalazine or methotrexate medication.
Newer medications include etanercept (enbrel) and infliximab (remicade). The early trials of these medications have been encouraging.
Lastly, surgical intervention may be required.
A rheumatologist would be able to help you better. The medications have to be tried to see which suits you the best. Sometimes 5 to 7 days courses of oral steroids are prescribed to get control over the disease.