""is this a matter of conditioning or is this something that usually happens when a pack doesn't fit properly or is packed wrong? ""

Welcome aboard!
To answer your question, it's a matter of both conditioning and ill fit sometimes.
Could you describe your pack and a rough description of how you are packing it? It's weight and your age/build/etc. will help too.
By day two, I'm feeling my pack, no matter it's fit or my conditioning...only because I don't live with a pack on my back all the time...but that's fairly normal (for us old poots <img src="/forums/images/graemlins/smirk.gif" alt="" />).