Is it more like muscle pain? The way you're describing it, it sounds like a bruise, but I'm not really sure from your description.

I definitely get muscle soreness if I let the weight of a pack ride on my back on the bike. When you're in riding position wearing a pack that was meant to be worn upright, the shoulder straps pull in a weird direction and the weight rests in a weird place. I do best if I loosen the shoulder straps all the way, just so there's enough tension to stabilize the pack, and allow all the weight of it to rest on the lower strap on my hipbones.