FWIW your gear combo is not a very heavy one so I would attribute it to posture (apart from weak core muscles) - review and see if you are in a habit of holding your camera close to your chest even while not shooting which leads to arching of lower back?
Also are your upper arms rested on your body while shooting? This is likely once the arms get tired and leads to the weight being transfered to an arched back...
Hope this helps
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