Back Pain when working, not when gaming for the same time. any advice?

So I‘ve been a gamer for all my life, never experienced back pain. Not while on the couch, not while pc gaming, never.

But now since I work on a computer in an office all day long, after not even 2 hours my back feels weird and i have the urge to stretch and bend and even crack it, even though i never did thos, not even after a wow all-nighter.

Any tips?