You provide a lot of useful information about your condition. I would think that your discectomy is not related to your current problem. Your symptoms of pain after sleeping for 5-6 hours is a little odd. If your pain is throughout the day and worst at night, you are likely more sensitive to it when you are not doing anything else. If it is ONLY at night, that is somewhat odd. I don't know how long you have had these symptoms for, but if it has been several months and you are ONLY having upper back pain, an MRI of your thoracic spine is not a bad idea. Your symptoms don't sound like classic AS. You cite some other potential causes for your pain and that is also reasonable to pursue if there are no imaging abnormalities. I would also think of what else you have been doing lately or has changed in your life that might contribute to the pain and if you could curtail any of these activities to see if you improve.