There always has to be one wiseacre in the crowd, LOL!

To the OP: The guy in the picture you posted has the butt way too high up on the bone, rather than buried down solid in the fleshy pocket of his shoulder like I and Oldtimr told you is the proper way. With that steel buttplate on his rifle I'd be surprised if he isn't in some real pain in the upper bony area after just a couple shots and it's certainly not the way to shoulder a rifle or shotgun. If you are equating that to what you are doing, there is your easy answer why you're hurting. I would not be shooting without having some padding up in your shoulder area, especially this warm time of the year when you might go the the range with just a light weight shirt on. The pad in the link the other member posted would be good or make your own, but especially remember to get the gun DOWN into your fleshy area and not up like in that picture. It also sounds like your problem is compounded by the makeshift benches at that private place you go to. You need to sit at the proper height to get everything into position so you can lean forward and have the butt into your shoulder and not above it.