monty, sumpin's wrong. Most of my shotguns are under 7 pounds and the ones I shoot most are about 6 1/4 pounds and I often shoot 1 oz loads.
I can shoot all day - hundreds of rounds - in my shirtsleeves and not get beaten up and I'm 70 yrs old.
How long have you been shooting and technically speaking, how good is your form? Then, how good is the fit of the gun?
It sounds to me that your shooting form is at fault and that, coincidentally, the gun fit is very poor.
This just shouldn't happen.
It happened to me just once, about 15 years ago when I shot a borrowed gun that was just silly regarding fit.