Driving for long distances will do it to me, even if I would 18 to 24 miles a week. The muscles and joints just get locked in that position over the duration. It's like body jet lag or something.
I have back pains from sitting too long - partially attributed to an injury I got while shoveling snow years ago.
It's probably all related to the back - if the doctor said it's okay, then it's okay.
We're getting older and that means we need to keep moving.