I will tell you why it hurts: Your muscles and tendons are used to that awful, forward-shoulder position. Putting them back works muscle and stretches tendons in ways they aren't used to, thus, it's gonna get sore.
I suggest you see a chiropractor. I dealt with posture issues and back pain for years, turns out it was from an old fall on my hip, putting my spine out of alignment. My tendons and muscles got used to the bad curvature of my spine, and it took months of physical and chiropractic therapy to get back to "normal" ("normal" because I honestly don't know what perfect posture feels like; I have some permanent damage, because tendons do *not* go back to normal once they are stretched out).
I did experience this, especially in the beginning: Muscles were very sore after being adjusted into the "correct" position. It takes the body a long time to heal, and I wouldn't be surprised if your spine is also out of alignment. That your shoulders naturally go forward makes me suspect that you don't have a very strong core; people with weak abs will hold their weight with their back, and that's what it looks like. Forward shoulders, forward neck (neck should rest against the headrest in the car, just look at all the drivers that hunch forward over their steering wheels...that's bad posture for ya).
Stumbled into Primal due to food allergies, and subsequent elimination of non-primal foods.
Start Gluten-Free/Soy-Free: December 2012; start weight 158lbs, Ladies size 6
Start Primal: March 2013, start weight 150lbs, Ladies size 6
Current: 132lbs, Ladies size 2
26lbs lost since cutting the crap.