Ever since I lost a fair bit of weight I’ve been having discomfort sitting. It feels as if my spine is less insulated by fat; I can’t lean back without feeling a significant amount of pressure on my spine. Could it be that fat on your back significantly protects your spine from pressure?
That’s true in my experience. I gained about 25 pounds (needed, I was painfully thin) and have found sitting on unupholstered chairs much more comfortable.
Rather than gain back your built-in cushion, you might try learning some yoga and/or other exercises designed to help you improve your posture and balance. If you strengthen your abdominal muscles, that will help you keep your spine straight, and if you build up the muscles along your spinal column, that will give you something to keep your spinal bones off the back of your chair.
Your spine might be straightening out now that you’ve reduced the load on it. You should adjust after some time. InternetLegend has a good idea, any kind of exercise might help. The first time I really ballooned up around the age of 30 and then lost the weight I was felt some odd aches and pains for a while.
Do a google search for Gokhale TED talk (I would do it for you, but I have a really flaky wifi connection here). Yes the extra fat was padding, but in a proper sitting position it’s not needed.
Instead of sitting on a triangle of sitz bones and tailbone, we should be sitting on the sitz bones and pelvic bone, which moves a lot of the spine away from the chair.