I think it should be called bike "tweaking" instead of bike fitting. It seems like something that should be done regularly, like oil changes. Because we change, the bike changes, etc.
Also, tension while riding does it to me. Also, sometimes I find myself with my head at a weird angle because of the sun/shade etc. and my shoulders hurt.
Finally, core, core, core. Google pilates and learn how to do hundreds. You can do that in 2 minutes in the morning. You can learn more core moves, but this one will work relatively quickly, and you can do it without going to the gym for instruction.
Posture is my sore point. I have curves in my spine...and I really have to contort into getting straighter.
good luck!