Doc, heavy loads in lighter shotguns do that. If I fired off a 1 1/4 oz Brenekke in a 6 lb shotgun, it'd hurt more than an oz load in a heavier gun.
A well kept secret,lots of folks into "Serious" shotgunning add those mag extensions, side saddles, lights, fuzzy dice and fender skirts to take the sting out, not to optimize the weapon.
My 9 lb plus HD 870 doesn't kick hard with anything. The 20 gauge 870 YE I got for the kids is vicious with oz loads.
Old threads here on cutting kick, wearable recoil pads, proper bench nethods,etc, can help you have more fun and less pain.
HTH....