I've been in the process of changing my posture after reading good books by Esther Gokhale for example.

I'm wondering if while consciously changing your posture you experienced any pain?

For example, my shoulders are slightly rounded and propel forward slightly. So I've been focusing on getting them more set back where they naturally should sit. Its the old standard move of raising the shoulders up, pulling them back slightly and dropping them into their natural place where they sit naturally. However, after keeping this position for some time my shoulder muscles start to ache strongly, my traps get really sore, etc.

Is this a good sign that I'm changing the body ('no pain no gain') or is this a bad sign?

Any personal experiences out there?