I agree with pretty much everything thats been said. I tend to shoot 4 rounds of skeet in a session. I use either a Parker 20 or a Merkel model 8 in 12, both with hard butt plates. The two most pertinent questions are:
1) Are you using low brass 2 3/4? There's no reason to shoot standard on the skeet range. Low brass is there for a reason.
2) How are you mounting your gun? Do you have it snug before you call for a bird? Or do hold it loose and then snap it up before calling the shot? I hunt quail, and so generally hold it low and snap it up. This means sloppier placement and more bruising. I find a nice pre-mounted position, while less effective for practice reduces this.
I don't have a problem with this though. I think its a combination of stance, grip on the gun, leaning into the recoil, and just getting used to it. I shoot a lot of clay targets. I wouldn't just go from nada to 4 rounds and say,"Gee, wonder why this hurts"? Work into it. I haven't shot a pump in years (before I became a high class redneck), and maybe the weight/angle of the 870 is wrong for you. But I would say go light, start with the gun mounted properly, and gradually work up to lots of shells, before buying somethig thats going to mess with your lenghth of pull etc. Also, there's no shame in a nice shoulder padded vest or jacket. There is zero application to SD, as the last thing you're going to be worried about is recoil. Just don't overshoot to the degree that you develop a flinch and you'll be fine.
FQ13