With apologies for crashing another of your DB threads this morning Jazzy...
I suspect you're experiencing that strange reality that kind of endears the instrument to those of us who play them... playing an upright bass is simply a very physical act. There are some things you can do to mitigate that. As in most things, your instructor is the best source for firsthand advice, but one thing I'll share;
Whether playing seated or standing, I was always taught to have the footrest adjusted so that the nut is at, or slightly above eye level. This keeps your arms and back in proper position. If I'm playing seated, I like a pretty high chair. It just feels more natural. Also to be mindful of the address of the instrument. Keep the left wrist generally straight. Keep the (vestigial on your Classico) rib of your bass tucked back there close to your ribs. This stabilizes the instrument, and keeps you in a comfortable playing position both at once.
Here's a picture of me cropped out from a New River Bound gig a couple years ago, that demonstrates surprisingly good posture and playing position. I look pretty relaxed here.