Oh man, @tarashah, tell me about it. I learned that lesson the hard way too, using a folding chair at my desk like some kind of monster. That sharp pain feels like a tiny angry person with an ice pick just setting up shop in your spine. It's wild how you just accept the suffering until you finally sit in something that doesn't hate you. My old chair should have come with a warning label and a free bottle of pain pills.
Yeah I used to just grab whatever stool was nearby for repair work. Then I started getting this sharp pain right between my shoulder blades after a couple hours. Finally switched to a decent chair with some lumbar support and the difference was crazy. Even a cheap office chair is way better than a kitchen stool, my back doesn't feel wrecked anymore.