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Have you done any work above or below where you experience the pain in your back?
Sounds like you are at the limits of your tissue so any deviation from ideal positioning is an issue.
Being more aware of the situation will help you to maintain proper positioning whatever the skill you are performing. Yes, it can seem like alot in the beginning, however, start chipping away at it. You will increase the time you are able to maintain proper positioning.
Have you video tapped when you are surfing or paddling so you can see exactly what is going on?
Sometimes you can think you are doing one thing when in reality you are doing another thing. This helps you see exactly what is going on when.
What are you doing to address your pelvis?