You are in a clinic and you are seeing a 62-year-old male who has presented with pain in the shoulder and pelvic area. On questioning, this patient reports that the pain has been present for 3 weeks, started on one side but is now bilateral and is now interfering with their sleep. The patient also reports that the morning stiffness lasts for at least 45 minutes and then resolves as the day goes on. You do some basic observations and the patient has a low grade fever. What is your main differential diagnosis?