Other possible reasons you hurt:
You're wearing high heels (felted wader boots), standing on a sloping bank all day, hunched forward.
You're waving your arm(s)in the air, which affects your center of gravity.
Your back and abdominal muscles are undertrained for this particular exertion. They act as a team to support your spine.
You're not taking in and eliminating sufficient water. But if you're an average American you probably have a pocket full of salty snacks.
You're not catching enough fish! The adrenaline and jubilation would wipe out any pain! :P