I am a physical therapist. Unfortunately although you have a number of previously diagnosed conditions, your lower back pain could be a separate condition. Lower back pain is very common. And, it sounds like sitting gives you some relief which generally signifies a mechanical cause for your pain. Spinal stenosis is aggravated with standing and walking and relieved with sitting and bending.
Make sure you see an orthopedic doctor that can help determine if you have a specific low back condition or if it is related to some of your other issues. If you have a back condition seek help from a physical therapist.
To learn more about different types of low back conditions, visit:
Hope this helps.