Is keeping good posture supposed to hurt at first? ?

I'm trying to practice good posture so I've been trying to sit up straight, shoulders and neck in line, etc. I've just started this today and I've noticed that it's really painful in the mid region of my back (somewhere between my thoracic and lumbar region). I've got my feet flat on the floor at a 90� angle and I'm sitting directly on the bottom of my pelvic bone to distribute weight evenly. I know it takes some getting used to but it's it actually supposed to HURT at first? it's not too bad just a typical sore-back-achy feeling. Am I doing it wrong? What's the right way? And if it is supposed to feel like this, how long on average should I expect it to last? Thank you yahoo friends!