Features are probably due to sacroilitis as the treating doctor has suspected. It usually has a relapsing remitting course. Since the pain is severe try taking NSAIDs like Etoricoxib for an extended duration of 2 weeks. Infra red or ultrasound therapy may give some relief. Kindly check Rheumatoid factor and ANA.
Next Steps
Shedding 5 plus kilograms may offer some benefit.
Minimum 30 minutes walking a day
Use a hard unyielding mattress like an ortho mattress of any reputed company to sleep on.